Fair value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Earning assets
Loans held for investment, net, difference	$ 2,105,495	$ 1,382,864
Tax indemnification receivable	168,011	259,532
Derivative instruments	0	0
Total earning assets, difference	2,105,495	1,382,864
Interest-bearing liabilities
Long-term borrowings, difference	(24,601)	7,874
Total interest-bearing liabilities, difference	(34,688)	(31,565)
Excess of net asset fair value over carrying value	2,070,807	1,351,299
Certificates of Deposit
Interest-bearing liabilities
Certificates of deposits, difference	(10,087)	(39,439)
Fair Value
Earning assets
Loans held for investment, net	20,673,136	16,520,786
Cash and cash equivalents	1,534,339	1,918,793
Available-for-sale investments	244,088	208,603
Accrued interest receivable	967,482	766,106
Tax indemnification receivable	168,011	259,532
Derivative instruments	812	7,808
Total	23,587,868	19,681,628
Interest-bearing liabilities
Short-term borrowings	0	0
Long-term borrowings	3,299,871	2,160,105
Accrued interest payable	35,363	21,058
Derivative instruments	2,584	25,937
Total interest-bearing liabilities	18,852,235	15,681,529
Fair Value | Money-market and savings accounts
Interest-bearing liabilities
Deposits	8,470,209	7,963,925
Fair Value | Certificates of Deposit
Interest-bearing liabilities
Deposits	7,044,208	5,510,504
Carrying Value
Earning assets
Loans held for investment, net	18,567,641	15,137,922
Cash and cash equivalents	1,534,339	1,918,793
Available-for-sale investments	244,088	208,603
Accrued interest receivable	967,482	766,106
Tax indemnification receivable	168,011	259,532
Derivative instruments	812	7,808
Total	21,482,373	18,298,764
Interest-bearing liabilities
Short-term borrowings	0	0
Long-term borrowings	3,275,270	2,167,979
Accrued interest payable	35,363	21,058
Derivative instruments	2,584	25,937
Total interest-bearing liabilities	18,817,547	15,649,964
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities
Deposits	8,470,209	7,963,925
Carrying Value | Certificates of Deposit
Interest-bearing liabilities
Deposits	$ 7,034,121	$ 5,471,065