Stock-Based Compensation Plans - PSUA Activity (Detail) (Performance Shares, USD $)	3 Months Ended
	Mar. 31, 2015
Performance Shares
Summary of Performance Share Unit Awards outstanding, Number of Units
Outstanding, Beginning Balance	134,341
Units granted	77,119	[1]
Units vested	(26,102)
Units forfeited	(2,679)
Outstanding, Ending Balance	182,679
Summary of Performance Share Unit Awards outstanding, Weighted Average Grant Date Fair Value
Outstanding, Beginning Balance, weighted average grant date fair value	$ 86.11
Granted, weighted average grant date fair value	$ 55.17	[1]
Vested, weighted average grant date fair value	$ 76.75
Forfeited, weighted average grant date fair value	$ 82.60
Outstanding, Ending Balance, weighted average grant date fair value	$ 74.44

[1] Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero percent and 150% of the performance units granted depending on the total shareholder return ranking compared to the peer companies at the end of the three-year performance period.