Debt (Junior Subordinated Debentures Outstanding) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018		Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Amount	$ 70,677		$ 70,384	[1]
Junior subordinated debenture, purchase accounting adjustment	40,200		40,400
Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Outstanding Amount	70,677		70,384	[2]
Initial Equity In Trust	$ 3,327	[3]	$ 3,327	[4]
BST I [Member]
Debt Instrument [Line Items]
Issue Date	Mar. 15, 2005		Mar. 15, 2005
Beginning Optional Redemption Date	Mar. 30, 2010		Mar. 30, 2010
Interest Rate Following Beginning Optional Redemption Date	3-month LIBOR+ 4.90%		3-month LIBOR+ 4.90%
Interest Rate	7.21%		6.59%
Debt Instrument, Maturity Date	Mar. 30, 2035		Mar. 30, 2035
BST I [Member] | Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Outstanding Amount	$ 14,766		$ 14,703	[2]
Initial Equity In Trust	$ 696	[3]	$ 696	[4]
BST I [Member] | Junior Subordinated Debentures [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on rate	4.90%		4.90%
BST II [Member]
Debt Instrument [Line Items]
Issue Date	May 04, 2005		May 04, 2005
Beginning Optional Redemption Date	Jul. 30, 2010		Jul. 30, 2010
Interest Rate Following Beginning Optional Redemption Date	3-month LIBOR + 4.85%		3-month LIBOR + 4.85%
Interest Rate	6.62%		6.23%
Debt Instrument, Maturity Date	Jul. 30, 2035		Jul. 30, 2035
BST II [Member] | Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Outstanding Amount	$ 16,539		$ 16,472	[2]
Initial Equity In Trust	$ 774	[3]	$ 774	[4]
BST II [Member] | Junior Subordinated Debentures [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on rate	4.85%		4.85%
BST III [Member]
Debt Instrument [Line Items]
Issue Date	May 10, 2005		May 10, 2005
Beginning Optional Redemption Date	Jul. 30, 2010		Jul. 30, 2010
Interest Rate Following Beginning Optional Redemption Date	3-month LIBOR + 4.85%		3-month LIBOR + 4.85%
Interest Rate	6.62%		6.23%
Debt Instrument, Maturity Date	Jul. 30, 2035		Jul. 30, 2035
BST III [Member] | Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Outstanding Amount	$ 6,696		$ 6,670	[2]
Initial Equity In Trust	$ 310	[3]	$ 310	[4]
BST III [Member] | Junior Subordinated Debentures [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on rate	4.85%		4.85%
BST IV [Member]
Debt Instrument [Line Items]
Issue Date	Apr. 24, 2006		Apr. 24, 2006
Beginning Optional Redemption Date	Jun. 30, 2011		Jun. 30, 2011
Interest Rate Following Beginning Optional Redemption Date	3-month LIBOR+ 4.85%		3-month LIBOR+ 4.85%
Interest Rate	7.16%		6.54%
Debt Instrument, Maturity Date	Jun. 30, 2036		Jun. 30, 2036
BST IV [Member] | Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Outstanding Amount	$ 9,843		$ 9,802	[2]
Initial Equity In Trust	$ 464	[3]	$ 464	[4]
BST IV [Member] | Junior Subordinated Debentures [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on rate	4.85%		4.85%
BST V [Member]
Debt Instrument [Line Items]
Issue Date	Jul. 21, 2006		Jul. 21, 2006
Beginning Optional Redemption Date	Sep. 30, 2011		Sep. 30, 2011
Interest Rate Following Beginning Optional Redemption Date	3-month LIBOR+ 4.85%		3-month LIBOR+ 4.85%
Interest Rate	7.16%		6.54%
Debt Instrument, Maturity Date	Sep. 30, 2036		Sep. 30, 2036
BST V [Member] | Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Outstanding Amount	$ 9,843		$ 9,802	[2]
Initial Equity In Trust	$ 464	[3]	$ 464	[4]
BST V [Member] | Junior Subordinated Debentures [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on rate	4.85%		4.85%
BST VI [Member]
Debt Instrument [Line Items]
Issue Date	Feb. 26, 2007		Feb. 26, 2007
Beginning Optional Redemption Date	Apr. 30, 2012		Apr. 30, 2012
Interest Rate Following Beginning Optional Redemption Date	3-month LIBOR+ 4.80%		3-month LIBOR+ 4.80%
Interest Rate	6.57%		6.18%
Debt Instrument, Maturity Date	Apr. 30, 2037		Apr. 30, 2037
BST VI [Member] | Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Outstanding Amount	$ 12,990		$ 12,935	[2]
Initial Equity In Trust	$ 619	[3]	$ 619	[4]
BST VI [Member] | Junior Subordinated Debentures [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on rate	4.80%		4.80%

[1] * See Note 2 for summary of adjustments.

[2] TrustOutstandingAmount ofJuniorSubordinated Debentures (1)InitialEquity In Trust (2)IssueDateBeginningOptionalRedemptionDateInterest RateFollowingBeginningOptionalRedemptionDateInterestRate atDecember 31,2017MaturityDateBST I$ 14,703$ 6963/15/20053/30/20103-month LIBOR+ 4.90%6.59%3/30/2035BST II 16,472 7745/4/20057/30/20103-month LIBOR + 4.85% 6.23%7/30/2035BST III 6,670 3105/10/20057/30/20103-month LIBOR + 4.85% 6.23%7/30/2035BST IV 9,802 4644/24/20066/30/20113-month LIBOR+ 4.85%6.54%6/30/2036BST V 9,802 4647/21/20069/30/20113-month LIBOR+ 4.85%6.54%9/30/2036BST VI 12,935 6192/26/20074/30/20123-month LIBOR+ 4.80%6.18%4/30/2037$ 70,384$ 3,327Amounts include purchase accounting adjustments which reduced the total carrying value by $40.4 million

[3] Amounts include purchase accounting adjustments which reduced the total carrying value by $40.2 million.

[4] Initial Equity in Trust is recorded as part of other assets in the unaudited Consolidated Balance Sheets.