Bank segment - Credit risk profile - payment activity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Credit risk profile based on payment activity for loans
Total past due	$ 27,855	$ 23,102
Current	4,650,289	4,720,517
Total financing receivables	4,678,144	4,743,619
Recorded Investment greater than 90 days and accruing	0	0
Residential 1-4 family
Credit risk profile based on payment activity for loans
Total past due	9,870	11,310
Current	2,056,153	2,036,741
Total financing receivables	2,066,023	2,048,051
Recorded Investment greater than 90 days and accruing	0	0
Commercial real estate
Credit risk profile based on payment activity for loans
Total past due	5,414	2,416
Current	740,169	797,979
Total financing receivables	745,583	800,395
Recorded Investment greater than 90 days and accruing	0	0
Home equity line of credit
Credit risk profile based on payment activity for loans
Total past due	3,480	2,986
Current	901,769	860,177
Total financing receivables	905,249	863,163
Recorded Investment greater than 90 days and accruing	0	0
Residential land
Credit risk profile based on payment activity for loans
Total past due	1,979	255
Current	16,632	18,634
Total financing receivables	18,611	18,889
Recorded Investment greater than 90 days and accruing	0	0
Commercial construction
Credit risk profile based on payment activity for loans
Total past due	0	0
Current	128,407	126,768
Total financing receivables	128,407	126,768
Recorded Investment greater than 90 days and accruing	0	0
Residential construction
Credit risk profile based on payment activity for loans
Total past due	0	0
Current	13,031	16,080
Total financing receivables	13,031	16,080
Recorded Investment greater than 90 days and accruing	0	0
Commercial loans
Credit risk profile based on payment activity for loans
Total past due	1,961	2,226
Current	587,708	689,825
Total financing receivables	589,669	692,051
Recorded Investment greater than 90 days and accruing	0	0
Consumer loans
Credit risk profile based on payment activity for loans
Total past due	5,151	3,909
Current	206,420	174,313
Total financing receivables	211,571	178,222
Recorded Investment greater than 90 days and accruing	0	0
30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	15,356	11,504
30-59 days past due | Residential 1-4 family
Credit risk profile based on payment activity for loans
Total past due	3,905	5,467
30-59 days past due | Commercial real estate
Credit risk profile based on payment activity for loans
Total past due	5,414	2,416
30-59 days past due | Home equity line of credit
Credit risk profile based on payment activity for loans
Total past due	1,936	1,263
30-59 days past due | Residential land
Credit risk profile based on payment activity for loans
Total past due	498	0
30-59 days past due | Commercial construction
Credit risk profile based on payment activity for loans
Total past due	0	0
30-59 days past due | Residential construction
Credit risk profile based on payment activity for loans
Total past due	0	0
30-59 days past due | Commercial loans
Credit risk profile based on payment activity for loans
Total past due	1,095	413
30-59 days past due | Consumer loans
Credit risk profile based on payment activity for loans
Total past due	2,508	1,945
60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	4,357	4,230
60-89 days past due | Residential 1-4 family
Credit risk profile based on payment activity for loans
Total past due	1,513	2,338
60-89 days past due | Commercial real estate
Credit risk profile based on payment activity for loans
Total past due	0	0
60-89 days past due | Home equity line of credit
Credit risk profile based on payment activity for loans
Total past due	177	381
60-89 days past due | Residential land
Credit risk profile based on payment activity for loans
Total past due	984	0
60-89 days past due | Commercial construction
Credit risk profile based on payment activity for loans
Total past due	0	0
60-89 days past due | Residential construction
Credit risk profile based on payment activity for loans
Total past due	0	0
60-89 days past due | Commercial loans
Credit risk profile based on payment activity for loans
Total past due	218	510
60-89 days past due | Consumer loans
Credit risk profile based on payment activity for loans
Total past due	1,465	1,001
Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	8,142	7,368
Greater than 90 days | Residential 1-4 family
Credit risk profile based on payment activity for loans
Total past due	4,452	3,505
Greater than 90 days | Commercial real estate
Credit risk profile based on payment activity for loans
Total past due	0	0
Greater than 90 days | Home equity line of credit
Credit risk profile based on payment activity for loans
Total past due	1,367	1,342
Greater than 90 days | Residential land
Credit risk profile based on payment activity for loans
Total past due	497	255
Greater than 90 days | Commercial construction
Credit risk profile based on payment activity for loans
Total past due	0	0
Greater than 90 days | Residential construction
Credit risk profile based on payment activity for loans
Total past due	0	0
Greater than 90 days | Commercial loans
Credit risk profile based on payment activity for loans
Total past due	648	1,303
Greater than 90 days | Consumer loans
Credit risk profile based on payment activity for loans
Total past due	$ 1,178	$ 963