Fair Value of Assets and Liabilities (Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale		$ 5,258,693	$ 5,223,302
Fixed maturities, trading, at fair value	[1]	39,650	38,793
Equity securities, at fair value	[1]	44,419	40,610
Other invested assets	[1]	304,417	263,074
Reinsurance recoverables		31,696,534	32,555,500
Receivables from parent and affiliates		280,118	300,116
Separate account assets		127,433,522	129,655,734
TOTAL ASSETS		169,396,445	172,459,576
Future policy benefits		17,431,953	18,593,130
Payables to parent and affiliates		240,129	228,210
Other Liabilities		1,013,952	1,060,123
TOTAL LIABILITIES		166,669,070	169,638,940
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		4,191,000	5,453,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Asset		931,000	823,000
Embedded Derivative, Fair Value of Embedded Derivative Gross Liability		5,122,000	6,276,000
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale		5,258,693	5,223,302
Fixed maturities, trading, at fair value		39,650	38,793
Equity securities, at fair value		44,419	40,610
Short-term investments		1,194	1,339
Cash equivalents			28,007
Other invested assets		10	8
Reinsurance recoverables		4,214,114	5,457,649
Receivables from parent and affiliates		129,358	132,571
Subtotal excluding separate account assets		9,687,438	10,922,279
Separate account assets		123,324,538	125,543,035
TOTAL ASSETS		133,011,976	136,465,314
Future policy benefits		4,191,373	5,452,583
Policyholders’ account balances		39,516	46,651
Payables to parent and affiliates		3,369	4,660
Other Liabilities		15,955	0
TOTAL LIABILITIES		4,250,213	5,503,894
Asset Netting		(87,981)	(115,086)
Liability Netting		(87,763)	(69,718)
Netting		200	45,400
Fair Value, Measurements, Recurring | Other invested assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Asset Netting		(87,981)	(115,086)
Fair Value, Measurements, Recurring | Payables to parent and affiliates
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability Netting		(87,763)	(69,718)
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		95,891	99,815
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		617,772	635,458
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		176,720	135,098
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		1,749,069	1,781,089
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		928,890	961,238
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		234,224	182,452
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		895,174	851,666
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		181,610	182,428
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		311,352	322,832
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		67,991	71,226
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
Fixed maturities, trading, at fair value		0	0
Equity securities, at fair value		101	94
Short-term investments		0	0
Cash equivalents			0
Other invested assets		0	0
Reinsurance recoverables		0	0
Receivables from parent and affiliates		0	0
Subtotal excluding separate account assets		101	94
Separate account assets		0	0
TOTAL ASSETS		101	94
Future policy benefits		0	0
Policyholders’ account balances		0	0
Payables to parent and affiliates		0	0
Other Liabilities		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 | Asset-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 | 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 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale		5,069,470	5,017,475
Fixed maturities, trading, at fair value		39,650	38,793
Equity securities, at fair value		27,279	22,991
Short-term investments		0	0
Cash equivalents			28,007
Other invested assets		87,981	115,094
Reinsurance recoverables		0	0
Receivables from parent and affiliates		122,827	132,571
Subtotal excluding separate account assets		5,347,207	5,354,931
Separate account assets		123,324,538	125,543,035
TOTAL ASSETS		128,671,745	130,897,966
Future policy benefits		0	0
Policyholders’ account balances		0	0
Payables to parent and affiliates		91,132	74,378
Other Liabilities		0	0
TOTAL LIABILITIES		91,132	74,378
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		74,322	80,611
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		617,772	635,458
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		176,554	134,924
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		1,748,392	1,779,935
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		871,184	901,080
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		234,224	182,243
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		880,942	837,766
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		87,126	71,400
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		311,159	322,832
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		67,795	71,226
Fair Value, Measurements, Recurring | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available-for-sale		189,223	205,827
Fixed maturities, trading, at fair value		0	0
Equity securities, at fair value		17,039	17,525
Short-term investments		1,194	1,339
Cash equivalents			0
Other invested assets		10	0
Reinsurance recoverables		4,214,114	5,457,649
Receivables from parent and affiliates		6,531	0
Subtotal excluding separate account assets		4,428,111	5,682,340
Separate account assets		0	0
TOTAL ASSETS		4,428,111	5,682,340
Future policy benefits		4,191,373	5,452,583
Policyholders’ account balances		39,516	46,651
Payables to parent and affiliates		0	0
Other Liabilities		15,955	0
TOTAL LIABILITIES		4,246,844	5,499,234
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		21,569	19,204
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		166	174
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		677	1,154
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		57,706	60,158
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		0	209
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		14,232	13,900
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		94,484	111,028
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		193	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		196	0
Other invested assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value investment measured at NAV per share		62,100	700
Separate Account Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value investment measured at NAV per share		$ 4,109,000	$ 4,113,000

[1] Prior period amounts have been reclassified to conform to current period presentation. See "Adoption of ASU 2016-01" in Note 2 for details