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Regulatory Matters (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Common Equity Tier One Capital $ 607,800 $ 571,663
Common Equity Tier One Capital Ratio 10.32% 10.00%
Common Equity Tier One Capital Required for Capital Adequacy $ 265,153 $ 257,222
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets 4.50% 4.50%
Tier One Risk Based Capital $ 758,089 $ 722,674
Tier One Risk Based Capital to Risk Weighted Assets 12.87% 12.64%
Tier One Risk Based Capital Required for Capital Adequacy $ 353,537 $ 342,962
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets 6.00% 6.00%
Capital $ 832,182 $ 794,584
Capital to Risk Weighted Assets 14.12% 13.90%
Capital Required for Capital Adequacy $ 471,383 $ 457,283
Capital Required for Capital Adequacy to Risk Weighted Assets 8.00% 8.00%
Tier One Leverage Capital $ 758,089 $ 722,674
Tier One Leverage Capital to Average Assets 9.34% 9.42%
Tier One Leverage Capital Required for Capital Adequacy $ 324,725 $ 306,848
Tier One Leverage Capital Required for Capital Adequacy to Average Assets 4.00% 4.00%
Basel III Minimum with Capital Conservation Buffer [Domain]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets [1] 7.00% 7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets [1] 8.50% 8.50%
Capital Required for Capital Adequacy to Risk Weighted Assets [1] 10.50% 10.50%
Tier One Leverage Capital Required for Capital Adequacy to Average Assets [1] 4.00% 4.00%
Boston Private Bank [Member]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Common Equity Tier One Capital $ 694,201 $ 661,991
Common Equity Tier One Capital Ratio 11.83% 11.64%
Common Equity Tier One Capital Required for Capital Adequacy $ 264,028 $ 256,030
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets 4.50% 4.50%
Common Equity Tier One Capital Required to be Well-Capitalized $ 381,373 $ 369,822
Common Equity Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets 6.50% 6.50%
Tier One Risk Based Capital $ 694,201 $ 661,991
Tier One Risk Based Capital to Risk Weighted Assets 11.83% 11.64%
Tier One Risk Based Capital Required for Capital Adequacy $ 352,037 $ 341,374
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets 6.00% 6.00%
Tier One Risk Based Capital Required to be Well Capitalized $ 469,382 $ 455,165
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets 8.00% 8.00%
Capital $ 767,576 $ 733,214
Capital to Risk Weighted Assets 13.08% 12.89%
Capital Required for Capital Adequacy $ 469,382 $ 455,165
Capital Required for Capital Adequacy to Risk Weighted Assets 8.00% 8.00%
Capital Required to be Well Capitalized $ 586,728 $ 568,956
Capital Required to be Well Capitalized to Risk Weighted Assets 10.00% 10.00%
Tier One Leverage Capital $ 694,201 $ 661,991
Tier One Leverage Capital to Average Assets 8.63% 8.70%
Tier One Leverage Capital Required for Capital Adequacy $ 321,920 $ 304,510
Tier One Leverage Capital Required for Capital Adequacy to Average Assets 4.00% 4.00%
Tier One Leverage Capital Required to be Well Capitalized $ 402,400 $ 380,637
Tier One Leverage Capital Required to be Well Capitalized to Average Assets 5.00% 5.00%
Boston Private Bank [Member] | Basel III Minimum with Capital Conservation Buffer [Domain]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Common Equity Tier One Capital Required for Capital Adequacy to Risk Weighted Assets [1] 7.00% 7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets [1] 8.50% 8.50%
Capital Required for Capital Adequacy to Risk Weighted Assets [1] 10.50% 10.50%
Tier One Leverage Capital Required for Capital Adequacy to Average Assets [1] 4.00% 4.00%
[1] Required capital ratios under the Basel III capital rules with the fully phased-in capital conservation buffer added to the minimum risk-based capital ratios. The fully phased-in ratios are effective for 2019, with lower requirements during the transition years 2016 through 2018.