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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Numerator for basic and diluted EPS — net income allocated to common stockholders	$ 15,300	$ 19,800	$ 8,000	$ 300	$ 15,800	$ 1,800	$ 15,500	$ 13,600	$ 43,388	$ 46,682	$ 42,092
Denominator for basic EPS — weighted average number of shares (shares)									30,354,227	30,134,430	30,553,460
Add — dilutive effect of:
Weighted average number of incremental shares issuable from restricted stock units (shares)									4,000	27,000	8,000
Denominator for diluted EPS — weighted average number of shares and dilutive potential shares (shares)									30,357,691	30,160,669	30,561,682
Earnings per share:
Basic (usd per share)	$ 0.51	$ 0.66	$ 0.27	$ 0.01	$ 0.53	$ 0.06	$ 0.52	$ 0.45	$ 1.43	$ 1.55	$ 1.38
Diluted (usd per share)	$ 0.51	$ 0.66	$ 0.27	$ 0.01	$ 0.53	$ 0.06	$ 0.52	$ 0.45	$ 1.43	$ 1.55	$ 1.38