Number of new stock issued during the period.

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Name:	us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Issuances of Common Stock - Issuances of Common Stock in an Overnight Offering (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Issuance of Common Stock
Net proceeds from issuance of common shares		$ 0	$ 704,938
Common stock
Issuance of Common Stock
Shares issued (in shares)	11,850,000
Underwriting discounts and offering costs	$ 29,800
Net proceeds from issuance of common shares	$ 704,900