Fair Value Measurements - Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Financial assets:
Held to maturity securities	$ 364,122		$ 345,088
Non-covered loans, net	6,090,034		5,789,313
Covered loans, net	188,269		255,714
Broker-dealer and clearing organization receivables	1,672,123		1,497,741
FDIC indemnification asset	33,143	$ 40,304	71,313	$ 73,351	$ 74,460	$ 91,648
Financial liabilities:
Broker-dealer and clearing organization payables	1,517,698		1,347,128
Carrying Amount
Financial assets:
Cash and cash equivalents	354,969		690,764
Securities purchased under agreements to resell	134,654		89,430
Assets segregated for regulatory purposes	207,336		180,993
Held to maturity securities	368,031		351,831
Loans held for sale	79,501		46,965
Non-covered loans, net	6,090,034		5,789,313
Covered loans, net	188,269		255,714
Broker-dealer and clearing organization receivables	1,672,123		1,497,741
FDIC indemnification asset	33,143		71,313
Other assets	64,907		62,904
Financial liabilities:
Deposits	7,663,447		7,063,811
Broker-dealer and clearing organization payables	1,517,698		1,347,128
Short-term borrowings	1,477,201		1,417,289
Debt	367,208		384,924
Other liabilities	6,630		3,708
Estimate of Fair Value
Financial assets:
Cash and cash equivalents	354,969		690,764
Securities purchased under agreements to resell	134,654		89,430
Assets segregated for regulatory purposes	207,336		180,993
Held to maturity securities	364,122		345,088
Loans held for sale	79,501		46,965
Non-covered loans, net	6,288,495		5,962,052
Covered loans, net	284,424		367,444
Broker-dealer and clearing organization receivables	1,672,123		1,497,741
FDIC indemnification asset	22,866		60,173
Other assets	64,907		62,904
Financial liabilities:
Deposits	7,662,392		7,058,837
Broker-dealer and clearing organization payables	1,517,698		1,347,128
Short-term borrowings	1,477,201		1,417,289
Debt	364,731		378,822
Other liabilities	6,630		3,708
Estimate of Fair Value | Level 1
Financial assets:
Cash and cash equivalents	354,969		690,764
Assets segregated for regulatory purposes	207,336		180,993
Estimate of Fair Value | Level 2
Financial assets:
Securities purchased under agreements to resell	134,654		89,430
Held to maturity securities	364,122		345,088
Loans held for sale	79,501		46,965
Non-covered loans, net	485,604		502,077
Broker-dealer and clearing organization receivables	1,672,123		1,497,741
Other assets	59,300		58,697
Financial liabilities:
Deposits	7,662,392		7,058,837
Broker-dealer and clearing organization payables	1,517,698		1,347,128
Short-term borrowings	1,477,201		1,417,289
Debt	364,731		378,822
Other liabilities	6,630		3,708
Estimate of Fair Value | Level 3
Financial assets:
Non-covered loans, net	5,802,891		5,459,975
Covered loans, net	284,424		367,444
FDIC indemnification asset	22,866		60,173
Other assets	$ 5,607		$ 4,207