Equity Compensation Plans - Summary of Unvested Restricted Shares (Detail) - Restricted Stock shares in Thousands	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Shares:
Nonvested at beginning of period (in shares) | shares	296
Granted (in shares) | shares	76
Vested (in shares) | shares	(139)
Forfeited/Cancelled (in shares) | shares	(14)
Nonvested at end of period (in shares) | shares	219
Weighted-Average Grant-Date Fair Value:
Nonvested at beginning of period (in dollars per share) | $ / shares	$ 34.01
Granted (in dollars per share) | $ / shares	45.18
Vested (in dollars per share) | $ / shares	22.63
Forfeited/Cancelled (in dollars per share) | $ / shares	38.95
Nonvested at end of period (in dollars per share) | $ / shares	$ 44.77