Income Taxes - Difference Detween U.S. Federal Statutory Income Tax Rate and Effective Income Tax Rate as a Percentage of Income (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	35.00%	(35.00%)	35.00%
Rate differential on VIEs	0.80%	11.90%	0.00%
State income taxes, net of federal benefit	3.10%	(5.60%)	3.40%
Permanent book/tax differences	2.50%	2.80%	0.90%
Other	(0.30%)	(1.50%)	(0.30%)
Effective income tax rate	41.10%	(27.40%)	39.00%