Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income to common shareholders	$ 46,719	$ 41,166	$ 97,465	$ 116,154
Net income allocated to participating securities - basic and diluted	1,101	840	2,334	2,541
Net income allocated to common shareholders - basic and diluted	$ 45,618	$ 40,326	$ 95,131	$ 113,613
Weighted-average common shares - basic	77,550,000	77,928,000	77,525,000	78,452,000
Dilutive potential common shares	127,000	147,000	128,000	157,000
Weighted-average common shares - diluted	77,677,000	78,075,000	77,653,000	78,609,000
Earnings per common share: Basic	$ 0.59	$ 0.52	$ 1.23	$ 1.45
Earnings per common share: Diluted	$ 0.59	$ 0.52	$ 1.23	$ 1.45
Weighted-average anti-dilutive potential common shares	317,893	761,113	578,863	815,765