Common Stock - Nonvested Restricted Shares (Detail) - $ / shares	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Granted					$ 52.85	$ 55.26	$ 36.40
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested Restricted Shares, Beginning Balance	471,979	479,103	485,981	286,051	286,051
Nonvested Restricted Shares, Granted	32,264	7,824	17,164	350,101
Restricted Shares, Vested	(1,427)	(14,948)	(13,008)	(150,171)
Nonvested Restricted Shares, Forfeited	(5,974)		(11,034)
Nonvested Restricted Shares, Ending Balance	496,842	471,979	479,103	485,981	496,842	286,051
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Nonvested, Weighted Average Grant Date Fair Value, Beginning of Period	$ 52.15	$ 52.07	$ 51.70	$ 42.35	$ 42.35
Granted	44.24	57.98	56.06	53.40
Vested	35.62	49.98	46.37	37.86
Forfeited	54.76		48.85
Nonvested, Weighted Average Grant Date Fair Value, End of Period	$ 51.65	$ 52.15	$ 52.07	$ 51.70	$ 51.65	$ 42.35