Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Parent Beginning Balance	$ 3,650,466	$ 2,966,514
Net Income Attributable to Parent	75,098	26,238
OCI, Net of Tax, Portion Attributable to Parent	10,345	23,831
Treasury Stock Purchases	(57,376)
Stockholders' Equity Attributable to Parent Ending Balance	3,680,840	3,020,184
Stockholders' Equity Attributable to Noncontrolling Interest Beginning Balance	245,626	220,607
Net income attributable to noncontrolling interest	927	1,051
OCI, Net of Tax, Portion Attributable to Noncontrolling Interest	(4,018)	12,035
Stockholders' Equity, Other	2,308	3,569
Stockholders' Equity Attributable to Noncontrolling Interest Ending Balance	242,536	233,661
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Beginning Balance	3,896,092	3,187,121
Net income	76,025	27,289
Other Comprehensive Income, Net of Tax	6,327	35,866
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Ending Balance	3,923,376	3,253,845
Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury Stock Purchases	(57,376)
Stockholders' Equity, Other	2,307	3,601
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury Stock Purchases	0
Stockholders' Equity, Other	$ 1	$ (32)
Class A Common Stock, $0.01 par value, one vote per share; 180,000,000 shares authorized at March 31, 2018 and December 31, 2017; 74,924,202 and 74,808,305 shares issued and 61,229,743 and 61,946,078 shares outstanding (net of 13,694,459 and 12,862,227 shares in treasury) on March 31, 2018 and December 31, 2017, respectively
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Repurchased During Period, Shares	792,921