Other Comprehensive Income - OCI - Reclassifications out of Accumulated Other Comprehensive Income (Loss)(Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassifications out of accumulated other comprehensive income (loss)
Other realized investment (gains) losses, net	$ (16,830)	$ (33,374)	$ (27,297)	$ (107,193)
Net investment income	(300,582)	(291,539)	(911,274)	(979,080)
Interest expense	(7,811)	(7,648)	(23,087)	(26,051)
Total before tax	(68,269)	(55,312)	(196,210)	(291,482)
Tax expense or benefit	(21,688)	(15,295)	(64,973)	(79,043)
Net income	46,581	40,017	131,237	212,439
Net of tax	(450)	(5,064)
Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Net of tax			2,108	(35,369)
Unrealized holding (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	(2,508)	(8,465)	(1,997)	(55,107)
Total before tax	(2,508)	(8,465)	(1,997)	(55,107)
Tax expense or benefit	(878)	(2,962)	(699)	(19,287)
Net income	1,630	5,503	1,298	35,820
Unrealized holding (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	(1,067)	(2,911)	(3,602)	(7,943)
Interest expense	690	1,092	2,357	1,675
Total before tax	(377)	(1,819)	(1,245)	(6,268)
Tax expense or benefit	(132)	(637)	(436)	(2,194)
Net income	245	1,182	809	4,074
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	2,193	2,494	6,485	6,962
Tax expense or benefit	(768)	(873)	(2,270)	(2,437)
Net of tax	1,425	1,621	4,215	4,525
Prior service (benefits) | Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Total before tax	109	150	(37)	452
Actuarial losses | Amount reclassified from accumulated other comprehensive income (loss)
Reclassifications out of accumulated other comprehensive income (loss)
Total before tax	$ 2,302	$ 2,644	$ 6,448	$ 7,414