Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_PreferredStockSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Related Party Transactions - Additional Information (Detail) - shares	Dec. 31, 2017	Sep. 05, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Preferred stock issued	0	0	0
Silver Creek [Member] \| Series A and B Preferred Stock [Member]
Related Party Transaction [Line Items]
Shares owned by employees and directors of the parent company (as a percent)	4.00%		7.00%	7.00%
Silver Creek [Member] \| Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Preferred stock issued	0		0