Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Net income available to common shareholders													$ 30,977	$ 34,938	$ 37,636
Weighted average number of shares used in calculating basic net income/(loss) per share													31,356,000	31,262,000	31,072,000
Stock options													57,000	104,000	44,000
Long-term incentive plan													223,000	144,000	93,000
Weighted average number of shares used in calculating diluted net income per share													31,636,000	31,510,000	31,209,000
Effect of stock-based compensation plans that were not included in the computation of diluted earnings per share because to do so would have been antidilutive
Average market price of common stock used for calculation of dilutive shares													$ 21.51	$ 23.44	$ 20.49
Basic	$ 0.27	$ 0.3	$ (1.08)	$ 1.5	$ (0.23)	$ 0.53	$ 0.28	$ 0.54	$ 0.66	$ 0.12	$ 0.25	$ 0.18	$ 0.99	$ 1.12	$ 1.21
Diluted	$ 0.26	$ 0.3	$ (1.08)	$ 1.49	$ (0.23)	$ 0.53	$ 0.28	$ 0.53	$ 0.66	$ 0.12	$ 0.25	$ 0.18	$ 0.97	$ 1.11	$ 1.21
Common Stock, shares outstanding	31,400,000				31,300,000				31,200,000				31,400,000	31,300,000	31,200,000