Securities Sold Under Repurchase Agreements (Details) - USD ($)	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	$ 977,714,000	$ 1,127,170,000
Collateralized securities, Fair Value of Securities Sold	976,400,000	1,134,418,000
Repurchase Borrowing, Balance of Liability	$ 827,772,000	$ 858,350,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	2.66%	2.79%
Average outstanding amount	$ 872,611,000	$ 893,830,000
Maximum amount outstanding at any month end	907,211,000	892,341,000
Overnight agreements
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	316,041,000	366,731,000
Collateralized securities, Fair Value of Securities Sold	317,799,000	370,704,000
Repurchase Borrowing, Balance of Liability	$ 250,702,000	$ 236,077,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.15%	0.16%
1 to 29 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	$ 10,199,000	$ 3,717,000
Collateralized securities, Fair Value of Securities Sold	10,114,000	3,781,000
Repurchase Borrowing, Balance of Liability	$ 9,798,000	$ 1,016,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.50%	0.45%
30 to 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	$ 4,388,000	$ 13,399,000
Collateralized securities, Fair Value of Securities Sold	4,356,000	13,628,000
Repurchase Borrowing, Balance of Liability	$ 4,320,000	$ 6,705,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	0.31%	0.38%
Over 90 days
Collateral Securities and Repurchase Borrowing agreements
Collateralized Securities, Book Value of Securities Sold	$ 647,086,000	$ 743,323,000
Collateralized securities, Fair Value of Securities Sold	644,131,000	746,305,000
Repurchase Borrowing, Balance of Liability	$ 562,952,000	$ 614,552,000
Repurchase Borrowing, Weighted Average Interest Rate (as a percent)	3.84%	3.83%

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 08
-Paragraph m
-Subparagraph 1
-Article 4

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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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-Name Regulation S-X (SX)
-Number 210
-Section 08
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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.

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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=34720828&loc=SL20225862-175312

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

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-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

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 55
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-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313

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-URI http://asc.fasb.org/extlink&oid=45278096&loc=d3e9760-107771

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
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-Number 210
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