Investment Securities - Proceeds from Calls and Sales, Realized Gains and Losses and Interest Income (Details)	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) item
Proceeds from calls and sales of investment securities
Proceeds from calls of available for sale securities	$ 0	$ 0	$ 121,200,000
Proceeds from sales of available for sale securities	0	0	825,400,000
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]
Gross realized gains on sales of investment securities	0	0	27,400,000
Gross realized losses on sales of investment securities	0	0	100,000
Provision for income taxes related to net realized gains on sale of investment securities	0	0	10,800,000
Interest income from taxable and nontaxable investment securities
Taxable interest income	106,600,000	102,300,000	83,000,000
Non-taxable interest income	$ 500,000	$ 100,000	$ 0
Non-taxable investment securities owned \| item			0

Represents the number of non-taxable investment securities owned.

No definition available.

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- Definition

The cash inflow associated with calls on debt securities classified as available-for-sale securities.

+ References

No definition available.

+ Details

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- References

No definition available.

+ Details

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- Definition

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- Definition

Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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- References

No definition available.

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- Definition

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities exempt from state, federal and other income tax.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.2)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

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- Definition

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities subject to state, federal and other income tax.

+ References

+ Details

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- Definition

Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e640-108580

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- References

No definition available.

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- Definition

Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3151-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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