Stock-based Compensation Plans - PSUA Activities (Detail) - Market-based TSR - $ / shares		9 Months Ended
		Sep. 30, 2017		Sep. 30, 2016
Summary of Market-based TSR Awards outstanding, Number of Units
Outstanding, Beginning Balance		395,908
Granted	[1]	358,519
Vested		(221,274)
Forfeited		(3,679)
Outstanding, Ending Balance		529,474
Summary of Market-based TSR Awards outstanding, Weighted Average Grant Date Fair Value
Outstanding, Beginning Balance, weighted average grant date fair value		$ 44.39
Granted, weighted average grant date fair value		26.26	[1]	$ 36.64
Vested, weighted average grant date fair value		43.01
Forfeited, weighted average grant date fair value		44.21
Outstanding, Ending Balance, weighted average grant date fair value		$ 32.69

[1] Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero 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