Stock-Based Compensation - Performance Based Units Earned under Long-Term Incentive Plan (Detail) - Performance stock units	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning Balance (in shares) | shares	163,967
Vested (in shares) | shares	(114,569)
Canceled (in shares) | shares	(2,309)
Accrued dividend equivalents (in shares) | shares	1,962
Ending Balance (in shares) | shares	49,051
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning Balance (in dollars per share)	$ 36.77
Earned (in dollars per share)	0.00
Vested (in dollars per share)	36.77
Canceled (in dollars per share)	36.77
Accrued dividend equivalents (in dollars per share)	36.77
Ending Balance (in dollars per share)	$ 36.76