Sale of Equity Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 27, 2019	Feb. 13, 2018
Equity [Abstract]
Issuance of common stock, shares	3,833,334	4,032,012
Common stock price per share	$ 150.00	$ 164.00
Proceeds from public offering of common stock, net of commissions and underwriting discounts and offering costs	$ 560.9	$ 631.2

Number of new stock issued during the period.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration