Deposits - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Banking and Thrift [Abstract]
Deposits	$ 17,873,293		$ 17,873,293		$ 15,505,383
Brokered deposits	9,500,000		9,500,000		8,200,000
Retail and other deposits	8,400,000		8,400,000		7,300,000
Stable interest-bearing deposits, total	5,600,000		5,600,000		5,500,000
Brokered deposit placement fee	4,000	$ 3,000	9,000	$ 7,000
Third party broker fees paid	6,000	$ 4,000	25,000	$ 10,000
Deposits exceeding FDIC insurance limits	420,000		420,000		396,000
Accrued interest on deposits	$ 52,000		$ 52,000		$ 28,000

Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e7018-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6935-107765

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Balance Type:	credit
Period Type:	instant