Other Comprehensive Income - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassifications out of accumulated other comprehensive income (loss)
Other realized investment (gains) losses, net		$ (97,845)	$ (84,832)	$ (151,705)
Net investment income		(1,276,559)	(1,254,430)	(1,228,388)
Interest expense		33,705	38,588	37,286
Total before tax		316,604	288,979	473,272
Tax expense or benefit		85,512	98,524	155,903
Net income		231,092	190,455	317,369
Net of tax		(42,243)	(26,482)	(31,449)
Unrealized holding gains / losses arising on fixed maturities, available-for-sale | Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Other realized investment (gains) losses, net		(63,022)	(40,217)	(47,751)
Total before tax		63,022	40,217	47,751
Tax expense or benefit		22,058	14,076	16,713
Net income		40,964	26,141	31,038
Unrealized holding gains / losses arising on cash flow hedges | Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Net investment income		(13,906)	(12,380)	(8,408)
Interest expense		2,657	0	0
Total before tax		11,249	12,380	8,408
Tax expense or benefit		3,937	4,333	2,943
Net income		7,312	8,047	5,465
Employee benefit plan adjustment
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI		9,281	11,856	7,776
Tax expense or benefit		(3,248)	(4,150)	(2,722)
Net of tax		6,033	7,706	5,054
Prior service costs (benefits)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI	[1]	601	694	(3,189)
Actuarial losses (gains)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI	[1]	9,882	12,550	2,730
Settlement
Reclassifications out of accumulated other comprehensive income (loss)
Reclassification from AOCI	[1]	$ 0	$ 0	$ 1,857

[1] These accumulated other comprehensive income components are included in the computation of net periodic (benefit) cost of employee benefit plans (see Note 17 for additional details).