Stockholders' Equity - Narrative (Detail) - USD ($)	1 Months Ended	3 Months Ended	6 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Jun. 30, 2019	Jun. 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)			697,174
Weighted-average grant date fair value (in dollars per share)			$ 434.17
Service period (in years)			3 years 7 months 28 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)				19,100
Issuance of common stock in public offering of common stock				$ 7,600,000
Sale of stock, remaining available for sale (in shares)					0
Sale of stock, equity offering agreement, authorized						$ 750,000,000.0
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)			722,361
Issuance of common stock in public offering of common stock			$ 348,200,000
Sale of stock, equity offering agreement, authorized					$ 750,000,000.0