Stock-based Compensation Plans - PGPS/RGPS Activities (Detail) - Performance-based PGPS/RGPS	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted | shares	122,921	[1]
Vested | shares	(20,231)
Outstanding, Ending Balance | shares	102,690	[1]
Granted, weighted average grant date fair value | $ / shares	$ 25.53	[1]
Vested, weighted average grant date fair value | $ / shares	25.64
Outstanding, Ending Balance, weighted average grant date fair value | $ / shares	$ 25.51

[1] Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero and 150% depending on achievement of specifically identified performance targets.