Benefit Plans - Summary of Company's Restricted Stock Units Activity (Detail) - Restricted Stock Units [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Beginning Balance	311,583	320,980	262,820
Number of Shares, granted	79,300	118,864	150,300
Number of Shares, vested	(89,915)	(120,721)	(87,840)
Number of Shares, cancelled/forfeited/expired	(80,320)	(7,540)	(4,300)
Number of Shares, Ending Balance	220,648	311,583	320,980
Weighted-Average Grant Date Fair Value, Beginning Balance	$ 29.78	$ 28.47	$ 28.22
Weighted-Average Grant Date Fair Value, granted	31.86	30.85	26.89
Weighted-Average Grant Date Fair Value, vested	27.97	27.30	24.98
Weighted-Average Grant Date Fair Value, cancelled/forfeited/expired	31.35	30.34	29.24
Weighted-Average Grant Date Fair Value, Ending Balance	$ 30.70	$ 29.78	$ 28.47
Aggregate Intrinsic Value	$ 5.6