Stockholders' Equity (Changes in Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss	$ (13,946)		$ (13,946)		$ (121,306)
Net other comprehensive income (loss) for the period	(39,878)	$ 26,025	(107,360)	$ (21,166)
Ending balance	4,551,499	3,824,137	4,551,499	3,824,137
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss					(121,306)	$ (129,292)
Other comprehensive income (loss) before reclassifications			106,303	73,056
Amounts reclassified from accumulated other comprehensive loss (income)			1,057	(51,890)
Net other comprehensive income (loss) for the period			(107,360)	(21,166)
Ending balance	(13,946)	(108,126)	(13,946)	(108,126)
Benefits Liability, Net of Tax [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss					28,945	(8,607)
Other comprehensive income (loss) before reclassifications			0	(252)
Amounts reclassified from accumulated other comprehensive loss (income)			1,057	661
Net other comprehensive income (loss) for the period			(1,057)	(409)
Ending balance	30,002	(8,198)	30,002	(8,198)
Cumulative Foreign Currency Exchange, Net of Tax [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss					(150,202)	(115,690)
Other comprehensive income (loss) before reclassifications			106,563	15,758
Amounts reclassified from accumulated other comprehensive loss (income)			0	0
Net other comprehensive income (loss) for the period			(106,563)	(15,758)
Ending balance	(43,639)	(99,932)	(43,639)	(99,932)
Net Unrealized Holdings Gains (Losses) on Investments, Net of Tax [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Accumulated other comprehensive loss					$ (49)	$ (4,995)
Other comprehensive income (loss) before reclassifications			(260)	57,550
Amounts reclassified from accumulated other comprehensive loss (income)			0	(52,551)
Net other comprehensive income (loss) for the period			260	(4,999)
Ending balance	$ (309)	$ 4	$ (309)	$ 4