Fair Value of Assets and Liabilities (Changes in Level 3 Assets and Liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	$ 17,525	$ 15,845
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	7
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	0
Fair Value, end of period	17,039	17,272
Equity securities | 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
Equity securities | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	(486)	1,420
Net unrealized investment gains (losses):
Included in earnings	(486)	1,420
Short-term Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	1,339
Total gains (losses) (realized/unrealized):
Net investment income	0
Purchases	3,400
Sales	0
Issuances	0
Settlements	(3,513)
Transfers into Level 3	0
Transfers out of Level 3	0
Other	(14)
Fair Value, end of period	1,194
Short-term Investments | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(18)
Net unrealized investment gains (losses):
Included in earnings	(18)
Short-term Investments | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0
Included in other comprehensive income (loss)	0
Net unrealized investment gains (losses):
Included in earnings	0
Other invested assets
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	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers into Level 3	9	8
Transfers out of Level 3	0	0
Other	0	0
Fair Value, end of period	10	19
Other invested assets | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	1	11
Net unrealized investment gains (losses):
Included in earnings	2	0
Other invested assets | 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
Reinsurance Recoverables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	5,457,649	5,474,263
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0
Net investment income	0	0
Purchases	219,457	216,595
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	4,214,114	5,080,379
Reinsurance Recoverables | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(1,462,992)	(610,479)
Net unrealized investment gains (losses):
Included in earnings	(1,409,320)	(574,024)
Reinsurance Recoverables | 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
Receivables from parents and affiliates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	0	6,493
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	(20)	0
Net investment income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers into Level 3	6,551	0
Transfers out of Level 3	0	(6,493)
Other	0	0
Fair Value, end of period	6,531	0
Receivables from parents and affiliates | 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
Receivables from parents and affiliates | 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
Future policy benefits
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(5,452,583)	(5,041,007)
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0
Net investment income	0	0
Purchases	0	0
Sales	0	0
Issuances	(226,345)	(214,741)
Settlements	0	0
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Other	0	0
Fair Value, end of period	(4,191,373)	(4,632,555)
Future policy benefits | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	1,487,555	623,193
Net unrealized investment gains (losses):
Included in earnings	1,433,872	586,992
Future policy benefits | 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
Policyholders' account balances
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	(46,651)	(20,337)
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0
Net investment income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	2,122	1,477
Transfers Into Level 3	0	0
Transfers out of Level 3	0	0
Other	0	0
Fair Value, end of period	(39,516)	(24,867)
Policyholders' account balances | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	5,013	(6,007)
Net unrealized investment gains (losses):
Included in earnings	5,013	(6,007)
Policyholders' account balances | 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 liabilities
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	8,825	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	(15,955)	0
Other liabilities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	(24,780)	0
Net unrealized investment gains (losses):
Included in earnings	(24,780)	0
Other liabilities | 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 | US government
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	19,204	0
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	0	0
Net investment income	0	0
Purchases	2,365	25
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Other	0	3,570
Fair Value, end of period	21,569	3,595
Available-for-sale | Fixed maturities | US government | 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 | US government | 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 | Foreign government bonds
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)	(8)
Net investment income	0
Purchases	0
Sales	0
Issuances	0
Settlements	0
Transfers into Level 3	0
Transfers out of Level 3	0
Other	174
Fair Value, end of period	166
Available-for-sale | Fixed maturities | Foreign government bonds | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	0
Available-for-sale | Fixed maturities | Foreign government bonds | Asset management fees and other income
Total gains (losses) (realized/unrealized):
Included in earnings	0
Net unrealized investment gains (losses):
Included in earnings	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	75,421	102,556
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	817	453
Net investment income	51	3
Purchases	1,022	5,958
Sales	0	(51,430)
Issuances	0	0
Settlements	(4,570)	(5,518)
Transfers into Level 3	0	1,814
Transfers out of Level 3	(209)	(1,802)
Other	15	(3,570)
Fair Value, end of period	72,615	48,816
Available-for-sale | Fixed maturities | Corporate securities | Realized investment gains (losses), net
Total gains (losses) (realized/unrealized):
Included in earnings	68	352
Net unrealized investment gains (losses):
Included in earnings	(26)	(62)
Available-for-sale | Fixed maturities | Corporate securities | 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 | Structured securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, beginning of period	111,028	19,856
Total gains (losses) (realized/unrealized):
Included in other comprehensive income (loss)	105	73
Net investment income	26	36
Purchases	190	4,000
Sales	0	0
Issuances	0	0
Settlements	(3,195)	(129)
Transfers into Level 3	13,513	64,507
Transfers out of Level 3	(26,794)	0
Other	0	0
Fair Value, end of period	94,873	88,343
Available-for-sale | Fixed maturities | Structured securities | 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 | Structured securities | 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