Fair value measurements - Fair Value Measurements on a Recurring Basis (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	$ 1,232,826	$ 1,388,533
Discount rate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Mortgage revenue bonds, measurement input	0.0341
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	$ 1,026,385	1,161,416
U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	117,787	154,349
Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	28,597	23,636
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, January 1	23,636	15,427
Principal payments received	0	0
Purchases	4,961	8,209
Unrealized gain (loss) included in other comprehensive income	0	0
Balance, December 31	28,597	23,636
Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	33	34
Fair value measurements on a recurring basis | Level 1 | Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	33	34
Fair value measurements on a recurring basis | Level 1 | Bank | Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Corporate bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 1 | Bank | Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 1 | Other | Interest rate swap
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 2
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	300	91
Derivative liabilities	2,185	596
Fair value measurements on a recurring basis | Level 2 | Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	1,204,229	1,364,897
Fair value measurements on a recurring basis | Level 2 | Bank | Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	297	91
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 2 | Bank | Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	3	0
Derivative liabilities	12	9
Fair value measurements on a recurring basis | Level 2 | Bank | Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	1,026,385	1,161,416
Fair value measurements on a recurring basis | Level 2 | Bank | U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	117,787	154,349
Fair value measurements on a recurring basis | Level 2 | Bank | Corporate bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	60,057	49,132
Fair value measurements on a recurring basis | Level 2 | Bank | Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 2 | Other | Interest rate swap
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	2,173	587
Fair value measurements on a recurring basis | Level 3
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | Bank
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	28,597	23,636
Fair value measurements on a recurring basis | Level 3 | Bank | Interest Rate Lock Commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	0
Derivative liabilities	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | U.S. Treasury and federal agency obligations
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | Corporate bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	0	0
Fair value measurements on a recurring basis | Level 3 | Bank | Mortgage revenue bonds
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	28,597	23,636
Fair value measurements on a recurring basis | Level 3 | Other | Interest rate swap
Fair value measurements on a recurring and nonrecurring basis
Derivative liabilities	$ 0	$ 0