Significant Accounting Policies 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]
Weighted average number of Shares Outstanding, Basic									48,413,138		48,402,059		48,382,860
Earnings Per Share, Basic	$ 0.41	$ 0.84	$ 1.17	$ 1.34	$ 0.57	$ 1.18	$ 1.55	$ 1.46	$ 3.75	[1]	$ 4.75	[1]	$ 4.41
Dilutive Securities, Effect on Basic Earnings Per Share, Including Options and Restrictive Stock Units									$ 0		$ 0		$ 0
Weighted Average Number of Shares, Restricted Stock									18,946		10,856		5,892
Per share diultive effect of restricted stock									$ 0.00		$ 0.00		$ 0.00
Dilutive Securities, Effect on Basic Earnings Per Share, Other									0		0		0
Incremental Common Shares Attributable to Share-based Payment Arrangements									49,861		65,712		80,767
Per share dilutive effect share based payment									$ 0.00		$ (0.01)		$ (0.01)
Net Income (Loss) Available to Common Stockholders, Diluted									$ 181,759		$ 229,870		$ 213,399
Weighted Average Number of Shares Outstanding, Diluted	48,481,945								48,481,945		48,478,627		48,469,519
Earnings Per Share, Diluted	$ 0.41	$ 0.84	$ 1.16	$ 1.34	$ 0.57	$ 1.18	$ 1.54	$ 1.46	$ 3.75		$ 4.74	[1]	$ 4.40

[1] (1) Basic and diluted net income per common share is computed independently for each of the periods presented. Accordingly, the sum of the quarterly basic and diluted net income per common share may not agree to the annual total.