Noninterest Income - Client Investment Fees (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gains and Losses on Derivatives [Abstract]
Sweep money market fees	$ 17,178	$ 6,474	$ 29,500	$ 10,870
Asset management fees	5,730	4,111	11,088	7,490
Repurchase agreement fees	6,544	2,397	11,739	3,648
Total client investment fees	$ 29,452	$ 12,982	$ 52,327	$ 22,008

Amount of revenue from providing investment advisory, asset management and administrative services, including, but not limited to, distribution and shareholder services, accounting, tax, legal, regulatory filing, share registration, and shareholder correspondence activities.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration