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
	Mar. 31, 2018	Mar. 31, 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	(486,510)	144,227
Reclassifications, net of tax	2,435	1,064
Other comprehensive income (loss)	(484,075)	145,291
Stockholders' equity, ending balance	5,820,301	4,744,827
Available for Sale Assets [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	1,569,289	692,314
Other comprehensive income (loss) before reclassifications, net of tax	(485,843)	140,700
Reclassifications, net of tax	(550)	(957)
Other comprehensive income (loss)	(486,393)	139,743
Stockholders' equity, ending balance	1,082,896	832,057
Deferred Acquisition Costs [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(8,547)	(6,682)
Other comprehensive income (loss) before reclassifications, net of tax	356	(500)
Reclassifications, net of tax	0	0
Other comprehensive income (loss)	356	(500)
Stockholders' equity, ending balance	(8,191)	(7,182)
Foreign Exchange [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	16,302	4,967
Other comprehensive income (loss) before reclassifications, net of tax	(1,023)	3,786
Reclassifications, net of tax	0	0
Other comprehensive income (loss)	(1,023)	3,786
Stockholders' equity, ending balance	15,279	8,753
Pension Adjustments [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(152,770)	(113,025)
Other comprehensive income (loss) before reclassifications, net of tax	0	241
Reclassifications, net of tax	2,985	2,021
Other comprehensive income (loss)	2,985	2,262
Stockholders' equity, ending balance	(149,785)	(110,763)
Total [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	1,424,274	577,574
Stockholders' equity, ending balance	$ 940,199	$ 722,865