Securities Sold Under Repurchase Agreements (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Collateral Securities and Repurchase Borrowing agreements
Average outstanding amount	$ 402,396,000	$ 759,458,000
Maximum amount outstanding at any month end	514,616,000	819,155,000
Collateralized Securities, Book Value of Securities Sold	460,681,000	690,151,000
Collateralized securities, Fair Value of Securities Sold	452,912,000	679,811,000
Repurchase Borrowing, Balance of Liability	$ 353,805,000	$ 504,985,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	1.32%	2.46%
Overnight agreements
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	$ 340,054,000	$ 295,282,000
Collateralized securities, Fair Value of Securities Sold	334,506,000	292,286,000
Repurchase Borrowing, Balance of Liability	$ 242,824,000	$ 190,915,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.25%	0.15%
1 to 29 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold		$ 10,250,000
Collateralized securities, Fair Value of Securities Sold		10,148,000
Repurchase Borrowing, Balance of Liability		$ 9,863,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)		0.50%
30 to 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	$ 109,300,000	$ 2,315,000
Collateralized securities, Fair Value of Securities Sold	107,238,000	2,317,000
Repurchase Borrowing, Balance of Liability	$ 100,000,000	$ 2,300,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	3.99%	0.33%
Over 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	$ 11,327,000	$ 382,304,000
Collateralized securities, Fair Value of Securities Sold	11,168,000	375,060,000
Repurchase Borrowing, Balance of Liability	$ 10,981,000	$ 301,907,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.74%	4.01%

Represents the average amount of securities sold under repurchase agreements during the period.

No definition available.

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- Definition

Represents the maximum balance outstanding at any month end during the period as of the balance sheet date.

+ References

No definition available.

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- Definition

Carrying amount of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Market value of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

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- Definition

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(a)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(1))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=49176635&loc=d3e9760-107771

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