Regulatory Capital Requirements (Group's and the Bank's actual capital amounts and ratios) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Group [Member]
Capital [Abstract]
Actual - Total capital	$ 910,828	$ 899,258
Actual - Tier 1 capital	852,882	842,133
Actual -Common equity tier 1 capital	652,012	644,804
Actual - Tier 1 leverage capital	852,882	842,133
Minimum Capital - Total Capital	359,130	353,653
Minimum capital - Tier 1 capital	269,348	265,240
Minimum capital - Common equity tier 1 capital	202,011	198,930
Minimum capital - Tier 1 leverage capital	242,395	242,057
Minimum to be well capitalized - Total Capital	448,913	442,067
Minimum to be well capitalized - Tier 1 capital	359,130	353,653
Minimum to be well capitalized - Common equity tier 1 capital	291,793	287,343
Minimum to be well capitalized - Tier 1 leverage	$ 302,994	$ 302,571
Risk Based Ratios [Abstract]
Capital to Risk Weighted Assets	20.29%	20.34%
Tier One Risk Based Capital to Risk Weighted Assets	19.00%	19.05%
Common Equity Tier OneTo Risk Weighted Assets	14.52%	14.59%
Tier One Leverage Capital to Average Assets	14.07%	13.92%
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	$ 888,557	$ 879,648
Actual - Tier 1 capital	830,845	822,776
Actual -Common equity tier 1 capital	830,845	822,776
Actual - Tier 1 leverage capital	830,845	822,776
Minimum Capital - Total Capital	358,832	353,265
Minimum capital - Tier 1 capital	269,124	264,949
Minimum capital - Common equity tier 1 capital	201,843	198,712
Minimum capital - Tier 1 leverage capital	241,559	241,417
Minimum to be well capitalized - Total Capital	448,541	441,581
Minimum to be well capitalized - Tier 1 capital	358,832	353,265
Minimum to be well capitalized - Common equity tier 1 capital	291,551	287,028
Minimum to be well capitalized - Tier 1 leverage	$ 301,948	$ 301,771
Risk Based Ratios [Abstract]
Capital to Risk Weighted Assets	19.81%	19.92%
Tier One Risk Based Capital to Risk Weighted Assets	18.52%	18.63%
Common Equity Tier OneTo Risk Weighted Assets	18.52%	18.63%
Tier One Leverage Capital to Average Assets	13.76%	13.63%
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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