Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income to common shareholders	$ 46,907	$ 34,829	$ 50,746	$ 74,988
Net income allocated to participating securities - basic and diluted	1,136	766	1,233	1,701
Net income allocated to common shareholders - basic and diluted	$ 45,771	$ 34,063	$ 49,513	$ 73,287
Weighted-average common shares - basic	77,523,000	77,951,000	77,512,000	78,719,000
Dilutive potential common shares	157,000	164,000	164,000	162,000
Weighted-average common shares - diluted	77,680,000	78,115,000	77,676,000	78,881,000
Earnings per common share: Basic	$ 0.59	$ 0.44	$ 0.64	$ 0.93
Earnings per common share: Diluted	$ 0.59	$ 0.44	$ 0.64	$ 0.93
Weighted-average anti-dilutive potential common shares	548,821	574,037	602,809	843,544