Regulatory Capital Requirements (Group's and the Bank's actual capital amounts and ratios) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Group [Member]
Capital [Abstract]
Actual - Total capital	$ 899,258	$ 876,657
Actual - Tier 1 capital	842,133	819,662
Actual -Common equity tier 1 capital	644,804	627,733
Actual - Tier 1 leverage capital	842,133	819,662
Minimum Capital - Total Capital	353,653	357,404
Minimum capital - Tier 1 capital	265,240	268,053
Minimum capital - Common equity tier 1 capital	198,930	201,040
Minimum capital - Tier 1 leverage capital	242,057	252,344
Minimum to be well capitalized - Total Capital	442,067	446,756
Minimum to be well capitalized - Tier 1 capital	353,653	357,404
Minimum to be well capitalized - Common equity tier 1 capital	287,343	290,391
Minimum to be well capitalized - Tier 1 leverage	$ 302,571	$ 315,430
Risk Based Ratios [Abstract]
Capital to Risk Weighted Assets	20.34%	19.62%
Tier One Risk Based Capital to Risk Weighted Assets	19.05%	18.35%
Common Equity Tier OneTo Risk Weighted Assets	14.59%	14.05%
Tier One Leverage Capital to Average Assets	13.92%	12.99%
Capital Required for Capital Adequacy to Risk Weighted Assets	8.00%	8.00%
Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets	6.00%	6.00%
Common Equity Tier One Required For Capital Adequacy To Risk Weighted Assets	4.50%	4.50%
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	4.00%	4.00%
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets	8.00%	8.00%
Tier One Risk Common Equity Tier One Required To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Bank [Member]
Capital [Abstract]
Actual - Total capital	$ 879,648	$ 857,259
Actual - Tier 1 capital	822,776	800,544
Actual -Common equity tier 1 capital	822,776	800,544
Actual - Tier 1 leverage capital	822,776	800,544
Minimum Capital - Total Capital	353,265	356,596
Minimum capital - Tier 1 capital	264,949	267,447
Minimum capital - Common equity tier 1 capital	198,712	200,585
Minimum capital - Tier 1 leverage capital	241,417	251,200
Minimum to be well capitalized - Total Capital	441,581	445,745
Minimum to be well capitalized - Tier 1 capital	353,265	356,596
Minimum to be well capitalized - Common equity tier 1 capital	287,028	289,734
Minimum to be well capitalized - Tier 1 leverage	$ 301,771	$ 314,000
Risk Based Ratios [Abstract]
Capital to Risk Weighted Assets	19.92%	19.23%
Tier One Risk Based Capital to Risk Weighted Assets	18.63%	17.96%
Common Equity Tier OneTo Risk Weighted Assets	18.63%	17.96%
Tier One Leverage Capital to Average Assets	13.63%	12.75%
Capital Required for Capital Adequacy to Risk Weighted Assets	8.00%	8.00%
Tier One Risk Based Capital Required For Capital Adequacy To Risk Weighted Assets	6.00%	6.00%
Common Equity Tier One Required For Capital Adequacy To Risk Weighted Assets	4.50%	4.50%
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	4.00%	4.00%
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier One Risk Based Capital Required To Be Well Capitalized To Risk Weighted Assets	8.00%	8.00%
Tier One Risk Common Equity Tier One Required To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%

The common equity tier 1 capital ratio as defined in the new regulatory framework.

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The amount of Comon Equity Tier 1 required to be categorized as well capitalized under the new regulatory framework.

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The minimum Common Equity Tier One Capital Ratio (Common Equity Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the new regulatory framework.

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Common Equity Tier 1 to Risk Weighted Assets (Common Equity Tier 1 divided by risk weighted assets) as defined by regulations.

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The minimum Common Equity Tier One Capital Ratio (Common Equity Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the new regulatory framework.

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The minimum amount of Common Equity Tier 1 required for capital adequacy purposes under the new regulatory framework.

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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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 minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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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 minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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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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