Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Earning assets
Loans held for investment, net, difference	$ 2,188,549	$ 2,105,495
Accrued interest receivable, difference	0
Tax indemnification receivable	54,941	168,011
Total earning assets, difference	2,188,549	2,105,495
Interest-bearing liabilities
Long-term borrowings, difference	28,967	(24,601)
Total interest-bearing liabilities, difference	49,499	(34,688)
Excess of net asset fair value over carrying value	2,238,048	2,070,807
Certificates of deposit
Interest-bearing liabilities
Deposits, difference	20,532	(10,087)
Fair Value
Earning assets
Loans held for investment, net	24,167,452	20,673,136
Cash and cash equivalents	1,839,054	1,534,339
Available-for-sale investments	172,370	244,088
Accrued interest receivable	1,270,026	967,482
Tax indemnification receivable	54,941	168,011
Derivative instruments	1,384	812
Total earning assets	27,505,227	23,587,868
Interest-bearing liabilities
Long-term borrowings	4,503,254	3,299,871
Accrued interest payable	62,627	35,363
Derivative instruments	454	2,584
Total interest-bearing liabilities	22,418,584	18,852,235
Fair Value | Money-market and savings accounts
Interest-bearing liabilities
Deposits	8,923,893	8,470,209
Fair Value | Certificates of deposit
Interest-bearing liabilities
Deposits	8,928,356	7,044,208
Carrying Value
Earning assets
Loans held for investment, net	21,978,903	18,567,641
Cash and cash equivalents	1,839,054	1,534,339
Available-for-sale investments	172,370	244,088
Accrued interest receivable	1,270,026	967,482
Tax indemnification receivable	54,941	168,011
Derivative instruments	1,384	812
Total earning assets	25,316,678	21,482,373
Interest-bearing liabilities
Long-term borrowings	4,532,221	3,275,270
Accrued interest payable	62,627	35,363
Derivative instruments	454	2,584
Total interest-bearing liabilities	22,468,083	18,817,547
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities
Deposits	8,923,893	8,470,209
Deposits, difference	0
Carrying Value | Certificates of deposit
Interest-bearing liabilities
Deposits	$ 8,948,888	$ 7,034,121