Dividend Restrictions - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Condensed Financial Statements, Captions [Line Items]
Dividend paid to the Parent Company	$ 380.0
Additional dividends that could have been paid to the Parent Company without regulatory approval	$ 463.4

This element represents disclosure of the total aggregate cash dividends paid to the entity by consolidated subsidiaries, by unconsolidated subsidiaries, and by 50% or less owned persons accounted for using the equity method.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(b))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

Name:	us-gaap_CashDividendsPaidToParentCompany
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration