Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 37,201	$ 43,190	$ 43,490	$ 38,845	$ 40,271	$ 45,599	$ 46,180	$ 41,756	$ 162,725	$ 173,806	$ 157,191
Earnings Per Share, Basic [Abstract]
Net income used in calculating basic earnings per share									160,054	169,156	152,285
Weighted average number of shares outstanding, basic									55,834	61,059	72,283
Earnings per share, basic	$ 0.67	$ 0.78	$ 0.76	$ 0.67	$ 0.68	$ 0.74	$ 0.73	$ 0.62	$ 2.87	$ 2.77	$ 2.11
Earnings Per Share, Diluted [Abstract]
Net income used in calculating diluted earnings per share									160,085	169,245	152,336
Weighted average number of shares outstanding, basic									55,834	61,059	72,283
Incremental common shares attributable to call options and warrants									787	1,342	824
Incremental common shares attributable to share-based payment arrangements									4	0	0
Weighed-average shares used in calculating diluted earnings per share									56,625	62,401	73,107
Earnings per share, diluted	$ 0.67	$ 0.78	$ 0.74	$ 0.65	$ 0.67	$ 0.72	$ 0.72	$ 0.61	$ 2.83	$ 2.71	$ 2.08
Performance Shares [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Participating Securities, Distributed and Undistributed Earnings									(2,671)	(4,650)	(4,906)
Performance Shares [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Participating Securities, Distributed and Undistributed Earnings									$ (2,640)	$ (4,561)	$ (4,855)