Stockholders' Equity - Summary of Stock Appreciation Rights Activity (Detail) (Stock appreciation rights [Member], USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Stock appreciation rights [Member]
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	0
Number of stock appreciation rights, Outstanding at March 31, 2015	661,316
Number of stock appreciation rights, Vested and exercisable at March 31, 2015	520,222
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	$ 0
Weighted Average Exercise Price, Outstanding at March 31, 2015	$ 24.08
Weighted Average Exercise Price, Vested and exercisable at March 31, 2015	$ 24.11
Weighted Average Remaining Contractual Term (in years), Outstanding at March 31, 2015	6 years 8 months 19 days
Weighted Average Remaining Contractual Term (in years), Vested and exercisable at March 31, 2015	6 years 8 months 16 days
Aggregate Intrinsic Value, Outstanding at March 31, 2015	$ 69.4
Aggregate Intrinsic Value, Vested and exercisable at March 31, 2015	$ 54.6