Bank segment - Allowance for credit losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	$ 77,084	$ 54,297	$ 53,355	$ 52,119
Charge-offs	(7,380)	(5,624)	(14,011)	(11,815)
Recoveries	770	2,064	1,788	3,563
Provision	10,833	7,688	20,734	14,558
Valuation allowance, balance at the end of the period	81,307	58,425	81,307	58,425
Total	5,450,802		5,450,802		$ 5,120,664
Ending balance: individually evaluated for impairment					2,691
Ending balance: collectively evaluated for impairment					50,664
Financing Receivables:
Loans and leases receivable before fees, gross	5,356,510		5,356,510		5,067,821
Ending balance: individually evaluated for impairment					40,737
Ending balance: collectively evaluated for impairment					5,079,927
Real estate
Allowance for credit losses:
Total	4,160,943		4,160,943		4,192,069
Real estate | Residential 1-4 family
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	4,476	1,911	2,380	1,976
Charge-offs	(7)	(5)	(7)	(19)
Recoveries	2	8	55	617
Provision	(560)	101	(667)	(559)
Valuation allowance, balance at the end of the period	3,911	2,015	3,911	2,015
Total	2,123,226		2,123,226		2,178,135
Ending balance: individually evaluated for impairment					898
Ending balance: collectively evaluated for impairment					1,482
Financing Receivables:
Ending balance: individually evaluated for impairment					15,600
Ending balance: collectively evaluated for impairment					2,162,535
Real estate | Commercial real estate
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	16,587	14,825	15,053	14,505
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	4,513	986	5,839	1,306
Valuation allowance, balance at the end of the period	21,100	15,811	21,100	15,811
Total	855,566		855,566		824,830
Ending balance: individually evaluated for impairment					2
Ending balance: collectively evaluated for impairment					15,051
Financing Receivables:
Loans and leases receivable before fees, gross					824,830
Ending balance: individually evaluated for impairment					1,048
Ending balance: collectively evaluated for impairment					823,782
Real estate | Home equity line of credit
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	6,225	6,493	6,922	6,371
Charge-offs	0	(19)	0	(19)
Recoveries	0	4	6	9
Provision	(11)	403	(173)	520
Valuation allowance, balance at the end of the period	6,214	6,881	6,214	6,881
Total	1,065,264		1,065,264		1,092,125
Ending balance: individually evaluated for impairment					322
Ending balance: collectively evaluated for impairment					6,600
Financing Receivables:
Ending balance: individually evaluated for impairment					12,073
Ending balance: collectively evaluated for impairment					1,080,052
Real estate | Residential land
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	352	425	449	479
Charge-offs	(343)	(4)	(351)	(4)
Recoveries	5	7	14	14
Provision	342	109	308	48
Valuation allowance, balance at the end of the period	356	537	356	537
Total	13,224		13,224		14,704
Ending balance: individually evaluated for impairment					0
Ending balance: collectively evaluated for impairment					449
Financing Receivables:
Ending balance: individually evaluated for impairment					3,091
Ending balance: collectively evaluated for impairment					11,613
Real estate | Commercial construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	3,446	2,843	2,097	2,790
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	1,311	(797)	2,371	(744)
Valuation allowance, balance at the end of the period	4,757	2,046	4,757	2,046
Total	92,904		92,904		70,605
Ending balance: individually evaluated for impairment					0
Ending balance: collectively evaluated for impairment					2,097
Financing Receivables:
Loans and leases receivable before fees, gross					70,605
Ending balance: individually evaluated for impairment					0
Ending balance: collectively evaluated for impairment					70,605
Real estate | Residential construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	14	3	3	4
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	0	(1)	(3)	(2)
Valuation allowance, balance at the end of the period	14	2	14	2
Total	10,759		10,759		11,670
Ending balance: individually evaluated for impairment					0
Ending balance: collectively evaluated for impairment					3
Financing Receivables:
Ending balance: individually evaluated for impairment					0
Ending balance: collectively evaluated for impairment					11,670
Commercial
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	12,977	10,814	10,245	9,225
Charge-offs	(699)	(494)	(1,068)	(1,112)
Recoveries	106	1,281	292	1,461
Provision	1,484	1,472	3,477	3,499
Valuation allowance, balance at the end of the period	13,868	13,073	13,868	13,073
Total	1,073,829		1,073,829		670,674
Ending balance: individually evaluated for impairment					1,015
Ending balance: collectively evaluated for impairment					9,230
Financing Receivables:
Loans and leases receivable before fees, gross					670,674
Ending balance: individually evaluated for impairment					8,418
Ending balance: collectively evaluated for impairment					662,256
Consumer
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	33,007	16,983	16,206	16,769
Charge-offs	(6,331)	(5,102)	(12,585)	(10,661)
Recoveries	657	764	1,421	1,462
Provision	3,754	5,415	9,582	10,490
Valuation allowance, balance at the end of the period	31,087	$ 18,060	31,087	$ 18,060
Total	$ 216,030		216,030		257,921
Ending balance: individually evaluated for impairment					454
Ending balance: collectively evaluated for impairment					15,752
Financing Receivables:
Ending balance: individually evaluated for impairment					507
Ending balance: collectively evaluated for impairment					$ 257,414
Impact of ASU No. 2016-13
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			19,441
Impact of ASU No. 2016-13 | Real estate | Residential 1-4 family
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			2,150
Impact of ASU No. 2016-13 | Real estate | Commercial real estate
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			208
Impact of ASU No. 2016-13 | Real estate | Home equity line of credit
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			(541)
Impact of ASU No. 2016-13 | Real estate | Residential land
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			(64)
Impact of ASU No. 2016-13 | Real estate | Commercial construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			289
Impact of ASU No. 2016-13 | Real estate | Residential construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			14
Impact of ASU No. 2016-13 | Commercial
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			922
Impact of ASU No. 2016-13 | Consumer
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period			$ 16,463