Stockholders' Equity - Summary of Stock Appreciation Rights Activity (Detail) - Jun. 30, 2015 - Equity Incentive Plan [Member] - Stock appreciation rights [Member] - USD ($) $ / shares in Units, $ in Millions	Total
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock appreciation rights, Outstanding at December 31, 2014	688,228
Number of stock appreciation rights, Granted	0
Number of stock appreciation rights, Exercised	(26,912)
Number of stock appreciation rights, Forfeited	(5,514)
Number of stock appreciation rights, Outstanding at June 30, 2015	655,802
Number of stock appreciation rights, Vested and exercisable at June 30, 2015	564,384
Weighted Average Exercise Price, Outstanding at December 31, 2014	$ 24.05
Weighted Average Exercise Price, Granted	0
Weighted Average Exercise Price, Exercised	23.20
Weighted Average Exercise Price, Forfeited	23.20
Weighted Average Exercise Price, Outstanding at June 30, 2015	24.09
Weighted Average Exercise Price, Vested and exercisable at June 30, 2015	$ 24.11
Weighted Average Remaining Contractual Term (in years), Outstanding at June 30, 2015	6 years 5 months 16 days
Weighted Average Remaining Contractual Term (in years), Vested and exercisable at June 30, 2015	6 years 5 months 16 days
Aggregate Intrinsic Value, Outstanding at June 30, 2015	$ 59.1
Aggregate Intrinsic Value, Vested and exercisable at June 30, 2015	$ 50.8