Earnings Per Share, Calculation of EPS (Details) (USD $) Share data in Thousands, except Per 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
Numerator:
Net income	$ 9,100,000	$ 29,200,000	$ 21,000,000	$ 26,500,000	$ 6,100,000	$ 4,100,000	$ 4,100,000	$ 10,800,000	$ 85,800,000		$ 25,100,000		$ 12,000,000
Less: Net income attributable to participating securities									0		0		(100,000)
Net income available to common stockholders									$ 85,800,000		$ 25,100,000		$ 11,900,000
Denominator - Weighted-average common shares outstanding (in thousands):
Basic									19,115	[1]	18,979	[1]	19,377	[1]
Diluted1									19,278	[2]	18,979	[2]	19,377	[2]
Earnings per common share, Basic:
Net income per share	$ 0.48	$ 1.52	$ 1.10	$ 1.39	$ 0.33	$ 0.21	$ 0.22	$ 0.57	$ 4.49		$ 1.32		$ 0.61
Earnings per common share, Diluted:
Net income per share1	$ 0.47	$ 1.51	$ 1.09	$ 1.38	$ 0.33	$ 0.21	$ 0.22	$ 0.57	$ 4.45	[2],[3]	$ 1.32	[2],[3]	$ 0.61	[2],[3]

[1] The basic weighted-average number of common shares outstanding during the period excludes unvested share-based payment awards.

[2] The diluted weighted-average number of common shares outstanding and diluted earnings per share for 2012 were calculated using the treasury method. The diluted weighted-average number of common shares outstanding and diluted earnings per share for 2011 and 2010 were calculated using the two-class method (see Note 1).

[3] Diluted weighted-average number of common shares outstanding and diluted earnings per share for 2012 is based on the treasury method. Diluted weighted-average number of common shares outstanding and diluted earnings per share for 2011 and 2010 are based on the two-class method (see Note 1 and Note 14).