Sale of Equity Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 13, 2018	Nov. 17, 2017
Equity [Abstract]
Issuance of common stock, shares	4,032,012	4,058,822
Common stock price per share	$ 164.00	$ 85.00
Proceeds from public offering of common stock, net of commissions and underwriting discounts and offering costs	$ 631.2	$ 325.8

Number of new stock issued during the period.

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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration