Stockholders' Equity - Narrative (Detail) - USD ($)	1 Months Ended	3 Months Ended			9 Months Ended
	Mar. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Aug. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock in public offering of common stock (in shares)	2,985,575
Issuance of common stock in public offering of common stock	$ 1,213,400,000		$ 1,213,434,000
Restricted stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, approved (in shares)					736,303
Weighted-average grant date fair value (in dollars per share)					$ 440.61
Service period (in years)					3 years 7 months 24 days
2017 ATM Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock in public offering of common stock (in shares)				612,422		631,522
Issuance of common stock in public offering of common stock				$ 265,700,000		$ 273,900,000
Sale of stock, equity offering agreement, authorized								$ 750,000,000.0
Sale of stock, remaining available for sale (in shares)							0
2018 ATM Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock in public offering of common stock (in shares)		181,194			903,555
Issuance of common stock in public offering of common stock		$ 99,400,000			$ 447,500,000
Sale of stock, equity offering agreement, authorized							$ 750,000,000.0