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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ (332,921)	$ 259,271	(819,431)	403,498
Reclassifications, net of tax	(3,779)	2,424	(1,344)	3,488
Other comprehensive income (loss)	(336,700)	261,695	(820,775)	406,986
Stockholders' equity, ending balance	5,571,609	5,053,416	5,571,609	5,053,416
Available for Sale Assets [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	1,082,896	832,057	1,569,289	692,314
Other comprehensive income (loss) before reclassifications, net of tax	(329,619)	257,595	(815,462)	398,295
Reclassifications, net of tax	(6,763)	403	(7,313)	(554)
Other comprehensive income (loss)	(336,382)	257,998	(822,775)	397,741
Stockholders' equity, ending balance	746,514	1,090,055	746,514	1,090,055
Deferred Acquisition Costs [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(8,191)	(7,182)	(8,547)	(6,682)
Other comprehensive income (loss) before reclassifications, net of tax	1,612	(473)	1,968	(973)
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	1,612	(473)	1,968	(973)
Stockholders' equity, ending balance	(6,579)	(7,655)	(6,579)	(7,655)
Foreign Exchange [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	15,279	8,753	16,302	4,967
Other comprehensive income (loss) before reclassifications, net of tax	(4,914)	2,149	(5,937)	5,935
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	(4,914)	2,149	(5,937)	5,935
Stockholders' equity, ending balance	10,365	10,902	10,365	10,902
Pension Adjustments [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(149,785)	(110,763)	(152,770)	(113,025)
Other comprehensive income (loss) before reclassifications, net of tax	0	0	0	241
Reclassifications, net of tax	2,984	2,021	5,969	4,042
Other comprehensive income (loss)	2,984	2,021	5,969	4,283
Stockholders' equity, ending balance	(146,801)	(108,742)	(146,801)	(108,742)
Total [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	940,199	722,865	1,424,274	577,574
Stockholders' equity, ending balance	$ 603,499	$ 984,560	$ 603,499	$ 984,560