Fair Value of Assets and Liabilities (Changes in Level 3 Assets and Liabilities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fixed Maturities Available-For-Sale | U.S. Treasury securities and obligations of U.S. government authorities and agencies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	$ 0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0
Net investment income	0
Purchases	15,634
Sales	0
Issuances	0
Settlements	0
Transfers into Level 3	0
Transfers out of Level 3	0
Other	3,570
Fair Value, end of period	19,204	$ 0
Fixed Maturities Available-For-Sale | U.S. Treasury securities and obligations of U.S. government authorities and agencies | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	0
Fixed Maturities Available-For-Sale | U.S. Treasury securities and obligations of U.S. government authorities and agencies | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	0
Fixed Maturities Available-For-Sale | Corporate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	102,556	95,492
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	1,590	(1,018)	$ 262
Net investment income	158	54	30
Purchases	18,022	5,459
Sales	(52,287)	(9,928)
Issuances	0	0
Settlements	(24,770)	(10,728)
Transfers into Level 3	42,125	29,881
Transfers out of Level 3	(6,193)	(6,379)
Other	(3,582)	0
Fair Value, end of period	75,421	102,556	95,492
Fixed Maturities Available-For-Sale | Corporate Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(2,198)	(277)	(1,533)
Net unrealized investment gains (losses):
Included in earnings	(2,736)	(560)	(1,392)
Fixed Maturities Available-For-Sale | Corporate Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Fixed Maturities Available-For-Sale | Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	19,856	173,347
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(3,178)	158	(939)
Net investment income	112	149	52
Purchases	108,731	21,473
Sales	(7,471)	(44,486)
Issuances	0	0
Settlements	(55,372)	(1,071)
Transfers into Level 3	78,159	48,957
Transfers out of Level 3	(34,008)	(177,780)
Other	0	0
Fair Value, end of period	111,028	19,856	173,347
Fixed Maturities Available-For-Sale | Asset-backed securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	4,199	(891)	42
Net unrealized investment gains (losses):
Included in earnings	0	(1,378)	0
Fixed Maturities Available-For-Sale | Asset-backed securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Fixed Maturities Available-For-Sale | Commercial Mortgage-Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0
Net investment income	0	0	0
Purchases	1,493	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	(1,493)	0
Other	0	0
Fair Value, end of period	0	0	0
Fixed Maturities Available-For-Sale | Commercial Mortgage-Backed Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Fixed Maturities Available-For-Sale | Commercial Mortgage-Backed Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Trading Account Assets | Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0
Net investment income	0	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	(527)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Other	0	559
Fair Value, end of period	0	0	0
Trading Account Assets | Asset-backed securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Trading Account Assets | Asset-backed securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	(32)	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Trading Account Assets | Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	15,770	18,248
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0
Net investment income	0	0	0
Purchases	0	0
Sales	0	(5,930)
Issuances	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Other	11	3,260
Fair Value, end of period	17,488	15,770	18,248
Trading Account Assets | Equity Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Trading Account Assets | Equity Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	1,707	192	2,207
Net unrealized investment gains (losses):
Included in earnings	2,345	(769)	2,162
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	75	165
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(38)	(90)	(245)
Net investment income	0	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Other	0	0
Fair Value, end of period	37	75	165
Equity Securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	337
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Equity Securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Short-term Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0
Net investment income	0
Purchases	8,425
Sales	(1)
Issuances	0
Settlements	(7,085)
Transfers into Level 3	0
Transfers out of Level 3	0
Other	0
Fair Value, end of period	1,339	0
Short-term Investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	0
Short-term Investments | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	0
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0	5,704
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0
Net investment income	0	(67)	0
Purchases	0	102
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers into Level 3	16	0
Transfers out of Level 3	(16)	(2,479)
Other	0	(3,260)
Fair Value, end of period	0	0	5,704
Other long-term investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	1,912
Net unrealized investment gains (losses):
Included in earnings	0	0	1,744
Other long-term investments | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Reinsurance Recoverables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	5,474,263	4,940,011
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0
Net investment income	0	0	0
Purchases	902,110	815,261
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Other	(760,101)	0
Fair Value, end of period	5,457,649	5,474,263	4,940,011
Reinsurance Recoverables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(158,623)	(281,009)	(635,006)
Net unrealized investment gains (losses):
Included in earnings	(315,998)	4,326,977	(482,828)
Reinsurance Recoverables | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Receivables from Parent and Affiliates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	6,493	5,000
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	16	(17)
Net investment income	0	0	0
Purchases	0	6,500
Sales	0	(1,987)
Issuances	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	(6,493)	(2,464)
Other	0	(559)
Fair Value, end of period	0	6,493	5,000
Receivables from Parent and Affiliates | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	(13)	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Receivables from Parent and Affiliates | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Future Policy Benefits
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(5,041,007)	(5,205,434)
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0
Net investment income	0	0	0
Purchases	0	0
Sales	0	0
Issuances	(875,008)	(811,396)
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Other	0	0
Fair Value, end of period	(5,452,583)	(5,041,007)	(5,205,434)
Future Policy Benefits | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	463,432	975,823	505,416
Net unrealized investment gains (losses):
Included in earnings	313,532	866,386	381,057
Future Policy Benefits | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0
Policyholders' Account Balances
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(20,337)	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0
Net investment income	0	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	4,677	(5,972)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Other	0	(5,902)
Fair Value, end of period	(46,651)	(20,337)	0
Policyholders' Account Balances | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(30,991)	(8,463)	0
Net unrealized investment gains (losses):
Included in earnings	(30,991)	(8,463)	0
Policyholders' Account Balances | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0
Net unrealized investment gains (losses):
Included in earnings	$ 0	$ 0	$ 0