Earnings Per Share Calculations (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share Calculations
Net income available to common stockholders	$ 29,151	$ 29,772	$ 41,588	$ 43,822
Weighted average common shares, basic	47,737,000	41,905,000	45,927,000	41,886,000
Dilutive shares	33,000		30,000
Shares used for dilutive computation	47,770,000	41,905,000	45,957,000	41,886,000
Net income per share - basic (in dollars per share)	$ 0.61	$ 0.71	$ 0.91	$ 1.05
Net income per share - diluted (in dollars per share)	$ 0.61	$ 0.71	$ 0.90	$ 1.05
Earnings per share calculations
Dilutive shares	33,000		30,000
Stock Appreciation Rights
Earnings Per Share Calculations
Dilutive shares			283,856
Earnings per share calculations
Anti-dilutive shares				283,856
Dilutive shares			283,856