Supplemental Information about Changes to Accumulated Other Comprehensive Income - Schedule of Analysis of Change in Balance by Component of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance			$ 4,566,861	$ 4,055,552
Other comprehensive income (loss) before reclassifications, net of tax	$ 61,035	$ 154,898	464,533	913,252
Reclassifications, net of tax	(6,293)	(1,229)	(2,805)	(2,373)
Other comprehensive income (loss)	54,742	153,669	461,728	910,879
Stockholders' equity, ending balance	5,167,685	5,086,383	5,167,685	5,086,383
Available for Sale Assets [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	1,090,055	1,084,551	692,314	332,333
Other comprehensive income (loss) before reclassifications, net of tax	55,160	154,389	453,455	911,069
Reclassifications, net of tax	(8,314)	(2,886)	(8,868)	(7,348)
Other comprehensive income (loss)	46,846	151,503	444,587	903,721
Stockholders' equity, ending balance	1,136,901	1,236,054	1,136,901	1,236,054
Deferred Acquisition Costs [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(7,655)	(8,658)	(6,682)	(5,115)
Other comprehensive income (loss) before reclassifications, net of tax	328	405	(645)	(3,138)
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	328	405	(645)	(3,138)
Stockholders' equity, ending balance	(7,327)	(8,253)	(7,327)	(8,253)
Foreign Exchange [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	10,902	8,331	4,967	3,627
Other comprehensive income (loss) before reclassifications, net of tax	5,547	104	11,482	4,808
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	5,547	104	11,482	4,808
Stockholders' equity, ending balance	16,449	8,435	16,449	8,435
Pension Adjustments [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(108,742)	(95,067)	(113,025)	(98,898)
Other comprehensive income (loss) before reclassifications, net of tax	0	0	241	513
Reclassifications, net of tax	2,021	1,657	6,063	4,975
Other comprehensive income (loss)	2,021	1,657	6,304	5,488
Stockholders' equity, ending balance	(106,721)	(93,410)	(106,721)	(93,410)
Total [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	984,560	989,157	577,574	231,947
Stockholders' equity, ending balance	$ 1,039,302	$ 1,142,826	$ 1,039,302	$ 1,142,826