Service Agreements and Related Party Transactions - Additional Information (Detail)			3 Months Ended		12 Months Ended
	Feb. 12, 2018 USD ($)	Oct. 14, 2014 Directors	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Related Party Transaction [Line Items]
Payment of stock offering fee			$ 2,500,000
WMMRC [Member]
Related Party Transaction [Line Items]
Administrative services agreement fee			110,000
Lawsuit against number of former directors | Directors		16
Litigation settlement amount							$ 37,000,000
Litigation settlement reserve for reimbursement of contingent fees and expenses							3,000,000
WMMRC [Member] | Investment Management Agreement And Administrative Services Agreement [Member]
Related Party Transaction [Line Items]
Expenses incurred under the agreements			$ 300,000	$ 300,000
WMMRC [Member] | Investment Management Agreement [Member]
Related Party Transaction [Line Items]
Description of fee			fee equal to the product of (x) the ending dollar amount of assets under management during the calendar month in question and (y) .002 divided by 12.
WMI Holdings Corp [Member]
Related Party Transaction [Line Items]
Litigation settlement reserve for reimbursement of contingent fees and expenses			$ 1,500,000
Other income					$ 123,000	$ 123,000	7,800,000
Income from litigation proceeds			$ 0
KKR Capital Markets LLC [Member] | Non-Exclusive Financial Advisor [Member]
Related Party Transaction [Line Items]
Transaction fee receive with respect to merger	$ 25,000,000
KKR Capital Markets LLC [Member] | Placement Agent [Member]
Related Party Transaction [Line Items]
Bridge financing facility, commitment fee percentage	0.25%
Fees payable for arranging bridge facility	$ 6,875,000
KKR Capital Markets LLC [Member] | Maximum [Member] | Placement Agent [Member]
Related Party Transaction [Line Items]
Bridge financing facility	$ 2,750,000,000
KKR Capital Markets LLC [Member] | Redeemable Convertible Series B Preferred Stock [Member]
Related Party Transaction [Line Items]
Payment of stock offering fee							8,250,000
Payment of additional deferred Fee							$ 8,250,000