Fair Value Measurements - Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financial assets:
Held to maturity securities	$ 332,388		$ 349,939
Non-covered loans, net	6,737,417		6,212,712
Covered loans, net	142,737		179,400
Broker-dealer and clearing organization receivables	1,491,507		1,464,378
FDIC indemnification asset	22,831	$ 23,525	29,340	$ 33,143	$ 40,304	$ 71,313
Financial liabilities:
Broker-dealer and clearing organization payables	1,396,401		1,287,563
Carrying Amount
Financial assets:
Cash and cash equivalents	406,150		487,382
Assets segregated for regulatory purposes	220,115		186,578
Securities purchased under agreements to resell	164,656		186,537
Held to maturity securities	348,163		355,849
Loans held for sale	137,358		133,754
Non-covered loans, net	6,737,417		6,212,712
Covered loans, net	142,737		179,400
Broker-dealer and clearing organization receivables	1,491,507		1,464,378
FDIC indemnification asset	22,831		29,340
Other assets	70,998		64,862
Financial liabilities:
Deposits	8,290,233		7,978,119
Broker-dealer and clearing organization payables	1,396,401		1,287,563
Short-term borrowings	1,216,649		1,206,424
Debt	287,204		275,821
Other liabilities	6,032		4,795
Estimate of Fair Value
Financial assets:
Cash and cash equivalents	406,150		487,382
Assets segregated for regulatory purposes	220,115		186,578
Securities purchased under agreements to resell	164,656		186,537
Held to maturity securities	332,388		349,939
Loans held for sale	137,358		133,754
Non-covered loans, net	6,833,717		6,406,757
Covered loans, net	217,503		269,386
Broker-dealer and clearing organization receivables	1,491,507		1,464,378
FDIC indemnification asset	22,831		20,122
Other assets	70,998		64,862
Financial liabilities:
Deposits	8,279,098		7,973,101
Broker-dealer and clearing organization payables	1,396,401		1,287,563
Short-term borrowings	1,216,649		1,206,424
Debt	284,921		289,719
Other liabilities	6,032		4,795
Estimate of Fair Value | Level 1
Financial assets:
Cash and cash equivalents	406,150		487,382
Assets segregated for regulatory purposes	220,115		186,578
Estimate of Fair Value | Level 2
Financial assets:
Securities purchased under agreements to resell	164,656		186,537
Held to maturity securities	332,388		349,939
Loans held for sale	137,358		133,754
Non-covered loans, net	593,276		577,889
Broker-dealer and clearing organization receivables	1,491,507		1,464,378
Other assets	69,940		59,053
Financial liabilities:
Deposits	8,279,098		7,973,101
Broker-dealer and clearing organization payables	1,396,401		1,287,563
Short-term borrowings	1,216,649		1,206,424
Debt	284,921		289,719
Other liabilities	6,032		4,795
Estimate of Fair Value | Level 3
Financial assets:
Non-covered loans, net	6,240,441		5,828,868
Covered loans, net	217,503		269,386
FDIC indemnification asset	22,831		20,122
Other assets	$ 1,058		$ 5,809