Other Comprehensive Income - Reclassifications out of AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassifications out of accumulated other comprehensive income (loss)
Other realized investment (gains) losses, net		$ (84,832)	$ (151,705)	$ 13,330
Net investment income		(1,254,430)	(1,228,388)	(1,091,389)
Tax expense or benefit		18,409	19,656	15,044
Net of tax		26,482	31,449	17,352
Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Net of tax		(26,482)	(31,449)
Unrealized holdings (gains) losses, net, arising on fixed maturities, available-for-sale
Reclassifications out of accumulated other comprehensive income (loss)
Net of tax		26,141	31,038	26,477
Unrealized holdings (gains) losses, net, 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		(40,217)	(47,751)
Total before tax		(40,217)	(47,751)
Tax expense or benefit		(14,076)	(16,713)
Net of tax		(26,141)	(31,038)
Unrealized holdings (gains) losses, net, arising on cash flow hedges
Reclassifications out of accumulated other comprehensive income (loss)
Net of tax		8,047	5,465	1,461
Unrealized holdings (gains) losses, net, arising on cash flow hedges | Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Net investment income		(12,380)	(8,408)
Total before tax		(12,380)	(8,408)
Tax expense or benefit		(4,333)	(2,943)
Net of tax		(8,047)	(5,465)
Amortization of employee benefit plan items
Reclassifications out of accumulated other comprehensive income (loss)
Net of tax		(7,706)	(5,054)	$ (10,586)
Amortization of employee benefit plan items | Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Total before tax		11,856	7,776
Tax expense or benefit		4,150	2,722
Net of tax		7,706	5,054
Prior service costs (benefits)	[1]	(694)	3,189
Actuarial gains (losses)	[1]	12,550	2,730
Settlement	[1]	$ 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).