Earnings Per Share (Computation of Earnings Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income to common shareholders	$ 71,177	$ 52,267	$ 143,652	$ 101,281
Net income allocated to participating securities - basic and diluted	1,328	1,166	2,694	2,322
Net income allocated to common shareholders - basic and diluted	$ 69,849	$ 51,101	$ 140,958	$ 98,959
Weighted-average common shares - basic	85,305	84,614	85,273	84,489
Dilutive potential common shares	178	253	178	266
Weighted-average common shares - diluted	85,483	84,867	85,451	84,755
Earnings per common share: Basic	$ 0.82	$ 0.60	$ 1.65	$ 1.17
Earnings per common share: Diluted	$ 0.82	$ 0.60	$ 1.65	$ 1.17