Fair Value of Assets and Liabilities (Changes in Level 3 Assets and Liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	$ 17,039	$ 17,272	$ 17,525	$ 15,845
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	(39)	0	(32)
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	0	0
Fair Value, end of period	16,625	17,350	16,625	17,350
Equity securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Equity securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	(414)	117	(900)	1,537
Net unrealized investment gains (losses):
Included in earnings	(414)	755	(900)	2,175
Short-term Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	1,194		1,339
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0		0
Net investment income	0		0
Purchases	1,870		5,270
Sales	0		0
Issuances	0		0
Settlements	(2,731)		(6,244)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Other	0		(14)
Fair Value, end of period	333		333
Short-term Investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0		(18)
Net unrealized investment gains (losses):
Included in earnings	(1)		(19)
Short-term Investments | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Cash equivalents
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
Net investment income	0		0
Purchases	2,550		2,550
Sales	0		0
Issuances	0		0
Settlements	(1,986)		(1,986)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Other	0		0
Fair Value, end of period	564		564
Cash equivalents | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Cash equivalents | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	10	19	0	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	9	8
Transfers out of Level 3	0	(8)	0	(8)
Other	0	0	0	0
Fair Value, end of period	5	0	5	0
Other invested assets | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(5)	(11)	(4)	0
Net unrealized investment gains (losses):
Included in earnings	(5)	0	(4)	0
Other invested assets | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Reinsurance Recoverables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	4,214,114	5,080,379	5,457,649	5,474,263
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	221,324	229,559	440,781	455,906
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	0	0
Fair Value, end of period	3,943,881	7,192,057	3,943,881	7,192,057
Reinsurance Recoverables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(491,557)	1,882,119	(1,954,549)	1,261,888
Net unrealized investment gains (losses):
Included in earnings	(441,257)	1,910,290	(1,844,015)	1,336,267
Reinsurance Recoverables | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Receivables from parents and affiliates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	6,531	0	0	6,493
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	(20)	0
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	6,551	0
Transfers out of Level 3	(6,531)	0	(6,531)	(6,493)
Other	0	0	0	0
Fair Value, end of period	0	0	0	0
Receivables from parents and affiliates | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Receivables from parents and affiliates | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Future policy benefits
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(4,191,373)	(4,632,555)	(5,452,583)	(5,041,007)
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	(228,243)	(217,897)	(454,588)	(432,638)
Settlements	0	0	0	0
Transfers Into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	0	0
Fair Value, end of period	(3,916,973)	(6,424,559)	(3,916,973)	(6,424,559)
Future policy benefits | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	502,643	(1,574,107)	1,990,198	(950,914)
Net unrealized investment gains (losses):
Included in earnings	459,185	(1,612,030)	1,879,579	(1,025,038)
Future policy benefits | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Policyholders' account balances
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(39,516)	(24,867)	(46,651)	(20,337)
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	5,429	(982)	7,551	495
Transfers Into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	0	0
Fair Value, end of period	(41,946)	(31,978)	(41,946)	(31,978)
Policyholders' account balances | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(7,859)	(6,129)	(2,846)	(12,136)
Net unrealized investment gains (losses):
Included in earnings	(7,860)	(6,129)	(2,847)	(12,136)
Policyholders' account balances | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Other liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(15,955)	0	0	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	8,880	0	17,705	0
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers Into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	0	0
Fair Value, end of period	(17,610)	0	(17,610)	0
Other liabilities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(10,535)	0	(35,315)	0
Net unrealized investment gains (losses):
Included in earnings	(10,590)	0	(35,231)	0
Other liabilities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Available-for-sale | Fixed maturities | US government
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	21,569	3,595	19,204	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0	0	0
Net investment income	0	0	0	0
Purchases	2,842	7,957	5,207	7,982
Sales	0	0	0	0
Issuances	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Other	0	0	0	3,570
Fair Value, end of period	24,411	11,552	24,411	11,552
Available-for-sale | Fixed maturities | US government | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Available-for-sale | Fixed maturities | US government | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Available-for-sale | Fixed maturities | Foreign government bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	166		0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(6)		(14)
Net investment income	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		174
Fair Value, end of period	160		160
Available-for-sale | Fixed maturities | Foreign government bonds | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Available-for-sale | Fixed maturities | Foreign government bonds | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0		0
Net unrealized investment gains (losses):
Included in earnings	0		0
Available-for-sale | Fixed maturities | Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	72,615	48,816	75,421	102,556
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(1,407)	1,045	(590)	1,498
Net investment income	84	56	135	59
Purchases	3,125	5,867	4,147	11,825
Sales	(201)	(767)	(201)	(52,197)
Issuances	0	0	0	0
Settlements	(15,501)	(7,135)	(20,071)	(12,653)
Transfers into Level 3	4,028	167	4,028	1,981
Transfers out of Level 3	0	(1,790)	(209)	(3,592)
Other	(1)	0	14	(3,570)
Fair Value, end of period	62,418	45,375	62,418	45,375
Available-for-sale | Fixed maturities | Corporate securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(324)	(884)	(256)	(532)
Net unrealized investment gains (losses):
Included in earnings	(49)	(1,046)	(75)	(1,108)
Available-for-sale | Fixed maturities | Corporate securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Available-for-sale | Fixed maturities | Structured securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	94,873	88,343	111,028	19,856
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(207)	(3,715)	(102)	(3,642)
Net investment income	2	52	28	88
Purchases	10,648	24,642	10,838	28,642
Sales	(196)	(7,471)	(196)	(7,471)
Issuances	0	0	0	0
Settlements	(4,871)	(42,671)	(8,066)	(42,800)
Transfers into Level 3	0	7,869	13,513	72,376
Transfers out of Level 3	(80,609)	(2,747)	(107,403)	(2,747)
Other	0	0	0	0
Fair Value, end of period	19,661	68,350	19,661	68,350
Available-for-sale | Fixed maturities | Structured securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	21	4,048	21	4,048
Net unrealized investment gains (losses):
Included in earnings	0	0	0	0
Available-for-sale | Fixed maturities | Structured securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0	0	0	0
Net unrealized investment gains (losses):
Included in earnings	$ 0	$ 0	$ 0	$ 0