Securities Financing Including Redeemable Preferred Stocks - Summary of Redeemable Preferred Stock (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders Equity [Line Items]
Accumulated dividends	$ 23,443	$ 27,761	$ 2,287
Series E Redeemable Preferred Stock [Member]
Stockholders Equity [Line Items]
Beginning balance	$ 145,029
Beginning balance, shares	139,768
Accretion relating to liquidation preference on preferred stock	$ 4,559
Accretion for discount and issuance costs on preferred stock	1,928
Accumulated dividends	23,443	27,761	$ 2,287
Ending balance	$ 174,959	$ 145,029
Ending balance, shares	139,768	139,768
Series F Redeemable Preferred Stock [Member]
Stockholders Equity [Line Items]
Beginning balance, shares	1
Ending balance, shares	1	1