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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 259,271	$ 455,396	403,498	758,355
Reclassifications, net of tax	2,424	(1,714)	3,488	(1,145)
Other comprehensive income (loss)	261,695	453,682	406,986	757,210
Stockholders' equity, ending balance	5,053,416	4,877,758	5,053,416	4,877,758
Available for Sale Assets [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	832,057	634,337	692,314	332,333
Other comprehensive income (loss) before reclassifications, net of tax	257,595	453,586	398,295	756,680
Reclassifications, net of tax	403	(3,372)	(554)	(4,462)
Other comprehensive income (loss)	257,998	450,214	397,741	752,218
Stockholders' equity, ending balance	1,090,055	1,084,551	1,090,055	1,084,551
Deferred Acquisition Costs [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(7,182)	(6,915)	(6,682)	(5,115)
Other comprehensive income (loss) before reclassifications, net of tax	(473)	(1,743)	(973)	(3,543)
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	(473)	(1,743)	(973)	(3,543)
Stockholders' equity, ending balance	(7,655)	(8,658)	(7,655)	(8,658)
Foreign Exchange [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	8,753	4,847	4,967	3,627
Other comprehensive income (loss) before reclassifications, net of tax	2,149	3,484	5,935	4,704
Reclassifications, net of tax	0	0	0	0
Other comprehensive income (loss)	2,149	3,484	5,935	4,704
Stockholders' equity, ending balance	10,902	8,331	10,902	8,331
Pension Adjustments [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(110,763)	(96,794)	(113,025)	(98,898)
Other comprehensive income (loss) before reclassifications, net of tax	0	69	241	514
Reclassifications, net of tax	2,021	1,658	4,042	3,317
Other comprehensive income (loss)	2,021	1,727	4,283	3,831
Stockholders' equity, ending balance	(108,742)	(95,067)	(108,742)	(95,067)
Total [Member]
Accumulated Other Comprehensive Income, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	722,865	535,475	577,574	231,947
Stockholders' equity, ending balance	$ 984,560	$ 989,157	$ 984,560	$ 989,157