Subsequent Events - Additional Information (Detail) - USD ($)	Jul. 10, 2018	Jul. 13, 2018	Jul. 05, 2018	Jun. 30, 2018	Dec. 31, 2017	Jan. 05, 2015
Subsequent Event [Line Items]
Offering amount deposited into escrow				$ 575,800,000	$ 578,900,000	$ 598,500,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends received	$ 10,400,000
Offering amount deposited into escrow		$ 1,700,000,000
Subsequent Event [Member] | WMMRC [Member]
Subsequent Event [Line Items]
Dividend declaration date	Jul. 10, 2018
Subsequent Event [Member] | WMMRC [Member] | Maximum [Member]
Subsequent Event [Line Items]
Net assets available for distribution from subsidairy			$ 11,100,000