Earnings Per Share (Computation of Earnings Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income to common shareholders	$ 323,770	$ 215,632	$ 149,296
Net income allocated to participating securities - basic and diluted	5,930	4,670	3,598
Net income allocated to common shareholders - basic and diluted	$ 317,840	$ 210,962	$ 145,698
Weighted-average common shares - basic	85,355	84,695	77,850
Dilutive potential common shares	166	268	99
Weighted-average common shares - diluted	85,521	84,963	77,949
Earnings per common share: Basic	$ 3.72	$ 2.49	$ 1.87
Earnings per common share: Diluted	$ 3.72	$ 2.48	$ 1.87