Summary of Significant Accounting Policies - Goodwill (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2013	Dec. 31, 2010
Goodwill [Line Items]
Goodwill	$ 34,817		$ 34,817	$ 32,168
Goodwill acquired during period	$ 0	$ 2,649
Gulf Coast
Goodwill [Line Items]
Goodwill impairment test, change in discount rate applied	3.25%
East Coast
Goodwill [Line Items]
Goodwill impairment test, change in discount rate applied	1.50%