Fair Value of Assets and Liabilities (Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	$ 5,223,302	$ 5,617,549
Trading account assets, at fair value	56,281	35,328
Equity securities, available-for-sale	23,122	16,756
Short-term investments	1,339	36,657
Other long-term investments	263,074	344,463
Reinsurance recoverables	32,555,500	28,674,226
Receivables from parent and affiliates	300,116	213,952
Separate account assets	129,655,734	116,606,428
TOTAL ASSETS	172,459,576	155,665,990
Payables to parent and affiliates	228,210	73,628
Total liabilities	169,638,940	153,169,187
Future Policy Benefits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net Liability	5,453,000	5,041,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Asset	823,000	1,157,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Liability	6,276,000	6,198,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	5,223,302	5,617,549
Trading account assets	56,281	35,328
Equity securities, available-for-sale	23,122	16,756
Short-term investments	1,339	36,657
Cash equivalents	28,007	7,642
Other long-term investments	8	77,865
Reinsurance recoverables	5,457,649	5,474,263
Receivables from parent and affiliates	132,571	137,637
Subtotal excluding separate account assets	10,922,279	11,403,697
Separate account assets	125,543,035	116,040,888
TOTAL ASSETS	136,465,314	127,444,585
Future policy benefits	5,452,583	5,041,007
Policyholders’ account balances	46,651	20,337
Payables to parent and affiliates	4,660	0
Total liabilities	5,503,894	5,061,344
Asset Netting	(115,086)	(13,019)
Liability Netting	(69,718)	(12,854)
Netting	45,400	200
Fair Value, Measurements, Recurring | Other long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Netting	(115,086)	(13,019)
Fair Value, Measurements, Recurring | Payables to parent and affiliates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liability Netting	(69,718)	(12,854)
Fair Value, Measurements, Recurring | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	99,815	160,740
Fair Value, Measurements, Recurring | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	635,458	626,486
Fair Value, Measurements, Recurring | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	135,098	108,782
Fair Value, Measurements, Recurring | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,781,089	2,361,518
Fair Value, Measurements, Recurring | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	961,238	884,284
Fair Value, Measurements, Recurring | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	182,452	221,848
Fair Value, Measurements, Recurring | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	851,666	599,016
Fair Value, Measurements, Recurring | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	38,793	19,256
Fair Value, Measurements, Recurring | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	182,428	189,016
Trading account assets, debt	0	302
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	322,832	382,671
Fair Value, Measurements, Recurring | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	71,226	83,188
Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, at fair value	17,488	15,770
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Trading account assets	0	0
Equity securities, available-for-sale	94	41
Short-term investments	0	31,007
Cash equivalents	0	5,644
Other long-term investments	0	0
Reinsurance recoverables	0	0
Receivables from parent and affiliates	0	0
Subtotal excluding separate account assets	94	36,692
Separate account assets	0	0
TOTAL ASSETS	94	36,692
Future policy benefits	0	0
Policyholders’ account balances	0	0
Payables to parent and affiliates	0	0
Total liabilities	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	0	0
Fair Value, Measurements, Recurring | Level 1 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Trading account assets, debt	0	0
Fair Value, Measurements, Recurring | Level 1 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, at fair value	0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	5,017,475	5,495,137
Trading account assets	38,793	19,558
Equity securities, available-for-sale	22,991	16,640
Short-term investments	0	5,650
Cash equivalents	28,007	1,998
Other long-term investments	115,094	90,884
Reinsurance recoverables	0	0
Receivables from parent and affiliates	132,571	131,144
Subtotal excluding separate account assets	5,354,931	5,761,011
Separate account assets	125,543,035	116,040,888
TOTAL ASSETS	130,897,966	121,801,899
Future policy benefits	0	0
Policyholders’ account balances	0	0
Payables to parent and affiliates	74,378	12,854
Total liabilities	74,378	12,854
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	80,611	160,740
Fair Value, Measurements, Recurring | Level 2 | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	635,458	626,486
Fair Value, Measurements, Recurring | Level 2 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	134,924	108,782
Fair Value, Measurements, Recurring | Level 2 | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,779,935	2,306,409
Fair Value, Measurements, Recurring | Level 2 | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	901,080	851,585
Fair Value, Measurements, Recurring | Level 2 | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	182,243	221,848
Fair Value, Measurements, Recurring | Level 2 | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	837,766	584,268
Fair Value, Measurements, Recurring | Level 2 | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	38,793	19,256
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	71,400	169,160
Trading account assets, debt	0	302
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	322,832	382,671
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	71,226	83,188
Fair Value, Measurements, Recurring | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, at fair value	0	0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	205,827	122,412
Trading account assets	17,488	15,770
Equity securities, available-for-sale	37	75
Short-term investments	1,339	0
Cash equivalents	0	0
Other long-term investments	0	0
Reinsurance recoverables	5,457,649	5,474,263
Receivables from parent and affiliates	0	6,493
Subtotal excluding separate account assets	5,682,340	5,619,013
Separate account assets	0	0
TOTAL ASSETS	5,682,340	5,619,013
Future policy benefits	5,452,583	5,041,007
Policyholders’ account balances	46,651	20,337
Payables to parent and affiliates	0	0
Total liabilities	5,499,234	5,061,344
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	19,204	0
Fair Value, Measurements, Recurring | Level 3 | Obligations of U.S. states and their political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	174	0
Fair Value, Measurements, Recurring | Level 3 | U.S. corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,154	55,109
Fair Value, Measurements, Recurring | Level 3 | U.S. corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	60,158	32,699
Fair Value, Measurements, Recurring | Level 3 | Foreign corporate public securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	209	0
Fair Value, Measurements, Recurring | Level 3 | Foreign corporate private securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	13,900	14,748
Fair Value, Measurements, Recurring | Level 3 | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, debt	0	0
Fair Value, Measurements, Recurring | Level 3 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	111,028	19,856
Trading account assets, debt	0	0
Fair Value, Measurements, Recurring | Level 3 | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets, at fair value	17,488	15,770
Other long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value investment measured at NAV per share	700	900
Separate Account Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value investment measured at NAV per share	$ 4,113,000	$ 566,000