Acquisitions and Divestitures - Purchase Price Allocation (Details) - USD ($) $ in Thousands	Apr. 03, 2017	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill		$ 236,335	$ 191,371	$ 191,307	$ 191,307
Coastal Securities
Business Acquisition [Line Items]
Cash and due from banks	$ 7,502
Interest-bearing cash	4,132
Trading securities	139,082
Loans held-for-sale	236,088
Investment securities	1,413
Other intangible assets, net	27,300
Premises and equipment, net	1,229
Other assets	1,672
Total assets acquired	418,418
Securities sold under agreements to repurchase	201,595
Other short-term borrowings	33,509
Fixed income payables	96,489
Other liabilities	316
Total liabilities assumed	331,909
Net Assets Acquired	86,509
Cash	(131,473)
Goodwill	44,964
As Acquired | Coastal Securities
Business Acquisition [Line Items]
Cash and due from banks	7,502
Interest-bearing cash	4,132
Trading securities	423,662
Loans held-for-sale	0
Investment securities	0
Other intangible assets, net	0
Premises and equipment, net	1,229
Other assets	1,658
Total assets acquired	438,183
Securities sold under agreements to repurchase	201,595
Other short-term borrowings	33,509
Fixed income payables	143,647
Other liabilities	958
Total liabilities assumed	379,709
Net Assets Acquired	58,474
Adjustment | Coastal Securities
Business Acquisition [Line Items]
Cash and due from banks	0
Interest-bearing cash	0
Trading securities	(284,580)
Loans held-for-sale	236,088
Investment securities	1,413
Other intangible assets, net	27,300
Premises and equipment, net	0
Other assets	14
Total assets acquired	(19,765)
Securities sold under agreements to repurchase	0
Other short-term borrowings	0
Fixed income payables	(47,158)
Other liabilities	(642)
Total liabilities assumed	(47,800)
Net Assets Acquired	$ 28,035