Fair Value Measurements Fair Value Nonrecurring Basis (Details) - Fair Value, Measurements, Nonrecurring [Member] - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		$ 1,778	[1]	$ 3,074	[2]
Other Real Estate Owned, Fair Value Disclosure	[3]			1,690
Assets, Fair Value Disclosure		43,832		39,541
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		0	[1]	0	[2]
Other Real Estate Owned, Fair Value Disclosure	[3]			0
Assets, Fair Value Disclosure		0		0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		0	[1]	0	[2]
Other Real Estate Owned, Fair Value Disclosure	[3]			0
Assets, Fair Value Disclosure		0		0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		1,778	[1]	3,074	[2]
Other Real Estate Owned, Fair Value Disclosure	[3]			1,690
Assets, Fair Value Disclosure		43,832		39,541
Portion at Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		6,388,297		6,021,611
Portion at Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		0		0
Portion at Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		0		0
Portion at Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		6,388,297		6,021,611
Changes Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure		(465)	[1]	(428)	[2]
Other Real Estate Owned, Fair Value Disclosure	[3]			(110)
Assets, Fair Value Disclosure		(25,366)		(10,066)
Boston Private Wealth [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[4]			34,777
Boston Private Wealth [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[4]			0
Boston Private Wealth [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[4]			0
Boston Private Wealth [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[4]			34,777
Boston Private Wealth [Member] | Changes Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[4]			$ (9,528)
Anchor [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[5]	42,054
Anchor [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[5]	0
Anchor [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[5]	0
Anchor [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[5]	42,054
Anchor [Member] | Changes Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, Fair Value Disclosure	[5]	$ (24,901)

[1] Collateral-dependent impaired loans held as of December 31, 2017 that had write-downs in fair value or whose specific reserve changed during 2017.

[2] Collateral-dependent impaired loans held as of December 31, 2016 that had write-downs in fair value or whose specific reserve changed during 2016

[3] OREO held as of December 31, 2016 that had write-downs in fair value during 2016.

[4] Goodwill balance at Boston Private Wealth as of December 31, 2016 that had write-downs in fair value during 2016. See Part II. Item 8. “Financial Statements and Supplementary Data - Note 8: Goodwill and Other Intangible Assets” for further detail.

[5] Goodwill balance at Anchor as of December 31, 2017 that had write-downs in fair value during 2017. The goodwill balance related to Anchor is recorded in other assets held for sale as of December 31, 2017. See Part II. Item 8. “Financial Statements and Supplementary Data - Note 8: Goodwill and Other Intangible Assets” for further detail. As of December 31, 2016 Fair value measurements at reporting date using: Gain (losses) from fair value changesQuoted prices in active marketsfor identicalassets (Level 1) Significant otherobservableinputs (Level 2) Significantunobservableinputs (Level 3) Year ended December 31, 2016(In thousands)Assets: Impaired loans (1)$3,074 $— $— $3,074 $(428)OREO (2)1,690 — — 1,690 (110)Goodwill (3)34,777 — — 34,777 (9,528) $39,541 $— $— $39,541 $(10,066)