Investment Securities - Gross Unrealized Losses and Fair Value, Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 626,721	$ 220,831
Less Than 12 Months, Unrealized Losses	(31,575)	(1,396)
12 Months or More, Fair Value	0	72,730
12 Months or More, Unrealized Losses	0	(2,172)
Fair Value, Total	626,721	293,561
Unrealized Losses	(31,575)	(3,568)
Corporate notes
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	321,303
Less Than 12 Months, Unrealized Losses	(20,208)
12 Months or More, Fair Value	0
12 Months or More, Unrealized Losses	0
Fair Value, Total	321,303
Unrealized Losses	(20,208)
Agency-guaranteed residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	305,418	104,861
Less Than 12 Months, Unrealized Losses	(11,367)	(656)
12 Months or More, Fair Value	0	66,579
12 Months or More, Unrealized Losses	0	(2,143)
Fair Value, Total	305,418	171,440
Unrealized Losses	$ (11,367)	(2,799)
Agency-guaranteed commercial real estate mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value		115,970
Less Than 12 Months, Unrealized Losses		(740)
12 Months or More, Fair Value		6,151
12 Months or More, Unrealized Losses		(29)
Fair Value, Total		122,121
Unrealized Losses		$ (769)