Stock-based Compensation Expense - Activity in Non-Performance Based Restricted Stock Awards (Detail) (Restricted Stock And Restricted Stock Units, USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock And Restricted Stock Units
Shares
Nonvested, beginning of period (in shares)	111,195
Granted (in shares)	44,811
Granted (in dollars per share)	$ 60.96
Vested (in shares)	(32,850)
Forfeited (in shares)	(17,868)
Nonvested, end of period (in shares)	105,288	111,195
Weighted Average Grant Date Fair Value
Nonvested, beginning of period (in dollars per share)	$ 59.32
Granted (in dollars per share)	$ 2.7	$ 3.4	$ 2.9
Vested (in dollars per share)	$ 60.75
Forfeited (in dollars per share)	$ 48.93
Nonvested, end of period (in dollars per share)	$ 61.34	$ 59.32