Stockholders' Equity - Additional Information (Detail) - USD ($)		3 Months Ended		12 Months Ended
	Feb. 14, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Aug. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash dividends declared (in dollars per share)	$ 2.28	$ 2.28	$ 2
Payments of dividends		$ 186,999,000	$ 148,083,000
Restricted stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividends payable		$ 2,200,000
Restricted stock units, approved (in shares)		546,375
Weighted-average grant date fair value (in dollars per share)		$ 379.83
Service period (in years)		3 years 6 months 21 days
At The Market (ATM) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Sale of stock, equity offering agreement, authorized					$ 750,000,000.0
Issuance of common stock in public offering of common stock, shares				763,201
Issuance of common stock in public offering of common stock, net				$ 355,100,000