Stockholders' Equity - Additional Information (Detail) - USD ($)				1 Months Ended	3 Months Ended				9 Months Ended
	Aug. 02, 2017	Apr. 26, 2017	Feb. 15, 2017	Mar. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Aug. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity offering agreement, authorized amount											$ 750,000,000.0
Stock issued and sold (in shares)				6,069,444
Stock issued, new issues				$ 2,126,300,000
Proceeds from issuance of stock				57,900,000
Payments of stock issuance costs				800,000
Cash dividends declared (in dollars per share)	$ 2.00	$ 2.00	$ 2.00		$ 2			$ 1.75	$ 6	$ 5.25
Payments of dividends					$ 159,500,000	$ 156,300,000	$ 148,100,000		$ 463,914,000	$ 374,151,000
Restricted stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, approved (in shares)									511,548
Weighted-average grant date fair value (in dollars per share)									3 years 5 months 24 days
Service period (in years)									$ 367.23
Special Distribution [Member] | Restricted stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividends payable				$ 2,600,000	$ 2,700,000	$ 2,700,000	$ 2,600,000		$ 2,700,000
Over-Allotment Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued and sold (in shares)				791,666