Bank segment - Loan modifications (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) contract	Sep. 30, 2016 USD ($) contract	Sep. 30, 2017 USD ($) contract	Sep. 30, 2016 USD ($) contract	Dec. 31, 2016 USD ($)
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			$ 21,797		$ 37,637
Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			11,592		14,450
Commercial real estate
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			1,281		1,346
Home equity line of credit
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			5,250		4,934
Residential land
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			1,555		2,751
Commercial construction
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			0		0
Residential construction
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			0		0
Commercial loans
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			2,052		14,146
Consumer loans
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			$ 67		$ 10
Troubled debt restructurings real estate loans
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	18	20	38	56
Pre-modification outstanding recorded investment	$ 1,275	$ 4,981	$ 3,072	$ 25,183
Post-modification outstanding recorded investment	1,275	4,981	3,066	25,296
Net increase in allowance	$ 222	$ 336	$ 387	$ 1,520
Troubled debt restructurings real estate loans | Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	2	2	7	11
Pre-modification outstanding recorded investment	$ 83	$ 251	$ 955	$ 2,239
Post-modification outstanding recorded investment	83	251	963	2,351
Net increase in allowance	$ 0	$ 46	$ 45	$ 305
Troubled debt restructurings real estate loans | Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	0	0	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Post-modification outstanding recorded investment	0	0	0	0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans | Home equity line of credit
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	15	12	28	30
Pre-modification outstanding recorded investment	$ 862	$ 1,268	$ 1,386	$ 2,705
Post-modification outstanding recorded investment	862	1,268	1,372	2,705
Net increase in allowance	$ 184	$ 237	$ 277	$ 492
Troubled debt restructurings real estate loans | Residential land
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	0	0	0	1
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 120
Post-modification outstanding recorded investment	0	0	0	121
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans | Commercial construction
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	0	0	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Post-modification outstanding recorded investment	0	0	0	0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans | Residential construction
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	0	0	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Post-modification outstanding recorded investment	0	0	0	0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans | Commercial loans
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	1	6	2	14
Pre-modification outstanding recorded investment	$ 330	$ 3,462	$ 672	$ 20,119
Post-modification outstanding recorded investment	330	3,462	672	20,119
Net increase in allowance	$ 38	$ 53	$ 38	$ 723
Troubled debt restructurings real estate loans | Consumer loans
Financing Receivable, Modifications [Line Items]
Number of contracts (in contracts) | contract	0	0	1	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 59	$ 0
Post-modification outstanding recorded investment	0	0	59	0
Net increase in allowance	$ 0	$ 0	$ 27	$ 0