Net Income Per Share Calculations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income available to common stockholders	$ 25,120	$ 33,650	$ 36,540	$ 45,344
Weighted average common shares outstanding, basic (in shares)	47,878,000	47,803,000	47,861,000	47,787,000
Dilutive SARs (treasury method) (in shares)	9,000	37,000	16,000	38,000
Weighed average common shares outstanding, dilutive (in shares)	47,887,000	47,840,000	47,877,000	47,825,000
Net income per share - basic (in dollars per share)	$ 0.52	$ 0.70	$ 0.76	$ 0.95
Net income per share - diluted (in dollars per share)	$ 0.52	$ 0.70	$ 0.76	$ 0.95
Net Income Per Share Calculations
Dilutive SARs (treasury method) (in shares)	9,000	37,000	16,000	38,000
Stock Appreciation Rights (SARs)
Earnings Per Share [Abstract]
Dilutive SARs (treasury method) (in shares)			65	185
Net Income Per Share Calculations
Dilutive SARs (treasury method) (in shares)			65	185