Earnings Per Share Earnings Per Share - Reconciliation (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, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings Per Share [Abstract]
Numerator for basic and diluted EPS — net income allocated to common stockholders	$ 15,100	$ 8,600	$ 2,200	$ 16,100	$ 16,100	$ 8,600	$ 21,500	$ (1,600)	$ 42,092	$ 44,578	$ 34,526
Denominator for basic EPS — weighted average number of shares									30,553,460	31,020,894	30,726,628
Add — dilutive effect of:
Weighted average number of incremental shares issuable from restricted stock units									8,222	13,923	49,406
Denominator for diluted EPS — weighted average number of shares and dilutive potential shares									30,561,682	31,034,817	30,776,034
Earnings per share:
Earnings Per Share, Basic	$ 0.50	$ 0.28	$ 0.07	$ 0.53	$ 0.53	$ 0.28	$ 0.69	$ (0.05)	$ 1.38	$ 1.44	$ 1.12
Diluted	$ 0.50	$ 0.28	$ 0.07	$ 0.53	$ 0.53	$ 0.28	$ 0.69	$ (0.05)	$ 1.38	$ 1.44	$ 1.12