Earnings Per Share and Capital Structure Computation of Basic and Diluted Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net income available to common shareholders	$ 19,259	$ 12,399	$ 19,005	$ 14,969
Weighted average common shares outstanding - Basic	39,356	39,431	39,296	39,426
Dilutive effect of stock-based awards	188	247	232	298
Weighted average common shares outstanding - Diluted	39,544	39,678	39,528	39,724
Earnings per common share - Basic	$ 0.49	$ 0.31	$ 0.48	$ 0.38
Earnings per common share - Diluted	$ 0.49	$ 0.31	$ 0.48	$ 0.38