Schedule of Information Related to Stock Repurchase Programs (Detail) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 30, 2017	Feb. 29, 2016	Dec. 31, 2014	Jul. 31, 2014
Class of Stock [Line Items]
Additional dollars authorized for repurchase		$ 1,200,000,000	$ 800,000,000		$ 400,000,000	$ 400,000,000		$ 400,000,000
Aggregate purchase price paid for shares purchased as part of publicly announced program		356,413,000	346,890,000	$ 224,260,000
Stock Repurchase Programs Twenty Fourteen
Class of Stock [Line Items]
Maximum number of dollars that may yet be purchased under the program							$ 342,050,000
Stock Repurchase Programs Twenty Fifteen
Class of Stock [Line Items]
Total number of shares purchased	[1]			1,819,503
Total number of shares purchased as part of publicly announced programs				1,326,207
Average price paid per share for shares purchased as part of publicly announced program				$ 125.34
Aggregate purchase price paid				$ 224,260,000
Aggregate purchase price paid for shares purchased as part of publicly announced program				166,222,000
Maximum number of dollars that may yet be purchased under the program				$ 175,828,000
Stock Repurchase Programs Twenty Sixteen
Class of Stock [Line Items]
Additional dollars authorized for repurchase			$ 400,000,000
Total number of shares purchased	[1]		2,983,320
Total number of shares cancelled			2,500
Average price paid per share for forfeited restricted shares			$ 0.01
Aggregate purchase price paid			$ 346,890,000
Maximum number of dollars that may yet be purchased under the program			$ 285,891,000
Stock Repurchase Programs Twenty Sixteen | Stock Repurchase Program
Class of Stock [Line Items]
Total number of shares purchased as part of publicly announced programs			2,512,592
Average price paid per share for shares purchased as part of publicly announced program			$ 115.39
Aggregate purchase price paid for shares purchased as part of publicly announced program			$ 289,937,000
Stock Repurchase Programs Twenty Seventeen
Class of Stock [Line Items]
Additional dollars authorized for repurchase		$ 400,000,000
Total number of shares purchased	[1]	3,266,121
Total number of shares cancelled		10,791
Average price paid per share for forfeited restricted shares		$ 0.01
Total number of shares purchased as part of publicly announced programs		2,960,843
Average price paid per share for shares purchased as part of publicly announced program		$ 108.83
Aggregate purchase price paid		$ 356,413,000
Aggregate purchase price paid for shares purchased as part of publicly announced program		322,231,000
Maximum number of dollars that may yet be purchased under the program		$ 363,660,000
Stock Repurchase Programs Twenty Seventeen | Stock Repurchase Program
Class of Stock [Line Items]
Total number of shares purchased as part of publicly announced programs		6,799,642
Average price paid per share for shares purchased as part of publicly announced program		$ 114.48
Aggregate purchase price paid for shares purchased as part of publicly announced program		$ 778,390,000
Stock Repurchase Programs Twenty Fifteen to Twenty Seventeen
Class of Stock [Line Items]
Additional dollars authorized for repurchase		$ 800,000,000
Total number of shares purchased	[1]	8,068,944
Total number of shares cancelled		13,291
Average price paid per share for forfeited restricted shares		$ 0.01
Aggregate purchase price paid		$ 927,563,000

[1] Includes 10,791 and 2,500 of restricted shares that were forfeited by former employees pursuant to the terms of our restricted stock purchase plan during 2017 and 2016, respectively.