Stockholder's Equity - Shares of Common Stock Issued During 2015 Under Employee Stock Purchase Plan (Detail) - $ / shares	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares available, Beginning balance			236,483	236,483
Shares issued during period (shares)	4,953	5,702	7,068
Shares available, Ending Balance	218,760			218,760
Shares issued during period (per share price)	$ 36.34	$ 31.38	$ 29.62