Regulatory Capital - Well Capitalized Regulatory Requirements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Actual Amount
Tier 1 Capital (to Average Assets)	$ 2,350,081	$ 2,011,583
Tier 1 Capital (to Risk-Weighted Assets)	2,350,081	2,011,583
Total Capital (to Risk-Weighted Assets)	2,597,926	2,197,997
Tier 1 Capital (to Average Assets)	$ 2,350,081	$ 2,011,583
Actual Ratio
Tier 1 Capital (to Average Assets)	11.90%	12.60%
Tier 1 Capital (to Risk-Weighted Assets)	11.90%	12.60%
Total Capital (to Risk-Weighted Assets)	13.10%	13.80%
Tier 1 Capital (to Average Assets)	11.00%	11.10%
Well Capitalized Regulatory Requirements, Amount
Tier 1 Capital (to Average Assets)	$ 1,288,435	$ 1,038,638
Tier 1 Capital (to Risk-Weighted Assets)	1,585,767	1,278,323
Total Capital (to Risk-Weighted Assets)	1,982,208	1,597,904
Tier 1 Capital (to Average Assets)	$ 1,067,739	$ 907,565
Well Capitalized Regulatory Requirements, Ratio
Tier 1 Capital (to Average Assets)	6.50%	6.50%
Tier 1 Capital (to Risk-Weighted Assets)	8.00%	8.00%
Total Capital (to Risk-Weighted Assets)	10.00%	10.00%
Tier 1 Capital (to Average Assets)	5.00%	5.00%

Common Equity Tier One Risk Based Capital

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Tier One Common Equity Capital Required to be Well Capitalized to Risk Weighted Assets

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Tier One Common Equity Capital to Risk Weighted Assets

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Tier One Risk Based Capital for Common Equity Required to be Capitalized

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The amount of total capital as defined in the regulations.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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