Selected Quarterly Financial Data (Unaudited) (Details) (USD $) 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
Selected Quarterly Financial Information [Abstract]
Revenue	$ 49,854	$ 44,939	$ 39,725	$ 39,407	$ 36,243	$ 32,962	$ 28,759	$ 27,742	$ 173,925	$ 125,706	$ 77,899
Operating expenses	20,268	18,893	15,363	16,515	15,342	11,274	11,839	9,226	71,039	47,681	40,973
Operating income	29,586	26,046	24,362	22,892	20,901	21,688	16,920	18,516	102,886	78,025	36,926
Net income attributable to CAI common stockholders	$ 17,411	$ 16,528	$ 15,135	$ 14,391	$ 12,891	$ 13,622	$ 10,880	$ 12,796	$ 63,465	$ 50,189	$ 28,383
Basic and diluted earnings per share available to CAI common stockholders [Abstract]
Basic (in dollars per share)	$ 0.87	$ 0.86	$ 0.78	$ 0.75	$ 0.67	$ 0.71	$ 0.56	$ 0.66	$ 3.26	$ 2.60	$ 1.58
Diluted (in dollars per share)	$ 0.85	$ 0.84	$ 0.77	$ 0.73	$ 0.66	$ 0.70	$ 0.55	$ 0.65	$ 3.18	$ 2.55	$ 1.56