Stockholders' Equity - Open Market Share Repurchases Activity (Detail) - USD ($)	1 Months Ended					9 Months Ended	12 Months Ended
	Sep. 30, 2017	Aug. 31, 2017	May 31, 2017	Mar. 31, 2017	Feb. 28, 2017	Sep. 30, 2017	Dec. 31, 2016	Jul. 31, 2017	Jun. 30, 2017	Apr. 30, 2017	Jan. 31, 2017
Condensed Financial Statements Captions [Line Items]
Total Number of Shares Purchased, beginning balance						33,837,526
Number of Shares Purchased	102,461	698,062	1,020,212	365,228	564,623
Total Number of Shares Purchased, ending balance	36,588,112					36,588,112	33,837,526
Average price of common stock purchased	$ 39.25	$ 41.77	$ 40.65	$ 45.15	$ 45.30	$ 45.88	$ 46.16
Dollar Value of Shares that may yet be Purchased under the Plans or Programs	$ 321,246,116	$ 325,268,111	$ 354,426,944	$ 395,903,537	$ 412,392,848	$ 321,246,116	$ 437,970,016	$ 354,426,944	$ 354,426,944	$ 395,903,537	$ 437,970,016
Publicly Announced Plans Or Programs
Condensed Financial Statements Captions [Line Items]
Total Number of Shares Purchased, beginning balance						33,837,526
Number of Shares Purchased	102,461	698,062	1,020,212	365,228	564,623
Total Number of Shares Purchased, ending balance	36,588,112					36,588,112	33,837,526