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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance			$ 6,231,421	$ 4,566,861
Other comprehensive income (loss) before reclassifications, net of tax	$ (128,498)	$ 61,035	(947,929)	464,533
Reclassifications, net of tax	2,623	(6,293)	1,279	(2,805)
Other comprehensive income (loss)	(125,875)	54,742	(946,650)	461,728
Stockholders' equity, ending balance	5,538,905	5,167,685	5,538,905	5,167,685
Available for Sale Assets [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	746,514	1,090,055	1,569,289	692,314
Other comprehensive income (loss) before reclassifications, net of tax	(129,296)	55,160	(944,758)	453,455
Reclassifications, net of tax	(362)	(8,314)	(7,675)	(8,868)
Other comprehensive income (loss)	(129,658)	46,846	(952,433)	444,587
Stockholders' equity, ending balance	616,856	1,136,901	616,856	1,136,901
Deferred Acquisition Costs [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(6,579)	(7,655)	(8,547)	(6,682)
Other comprehensive income (loss) before reclassifications, net of tax	930	328	2,898	(645)
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	930	328	2,898	(645)
Stockholders' equity, ending balance	(5,649)	(7,327)	(5,649)	(7,327)
Foreign Exchange [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	10,365	10,902	16,302	4,967
Other comprehensive income (loss) before reclassifications, net of tax	(132)	5,547	(6,069)	11,482
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	(132)	5,547	(6,069)	11,482
Stockholders' equity, ending balance	10,233	16,449	10,233	16,449
Pension Adjustments [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(146,801)	(108,742)	(152,770)	(113,025)
Other comprehensive income (loss) before reclassifications, net of tax	0	0	0	241
Reclassifications, net of tax	2,985	2,021	8,954	6,063
Other comprehensive income (loss)	2,985	2,021	8,954	6,304
Stockholders' equity, ending balance	(143,816)	(106,721)	(143,816)	(106,721)
Total [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	603,499	984,560	1,424,274	577,574
Stockholders' equity, ending balance	$ 477,624	$ 1,039,302	$ 477,624	$ 1,039,302