Fair value measurements - Fair Value Measurements on a Recurring Basis (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) instrument	Dec. 31, 2019 USD ($)
Fair value measurements on a recurring and nonrecurring basis
Available-for-sale, at fair value	$ 1,970,417	$ 1,232,826
Discount rate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Mortgage revenue bonds, measurement input	0.0213
Mortgage revenue bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, January 1	$ 28,597	23,636
Principal payments received	(1,641)	0
Purchases	229	4,961
Unrealized gain (loss) included in other comprehensive income	0	0
Balance, December 31	$ 27,185	28,597
Mortgage revenue bonds | Bank
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Number of mortgage revenue bonds held | instrument	2
Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	$ 0	0
Derivative liabilities	500	33
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
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	500	33
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	4,536	300
Derivative liabilities	4,530	2,185
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	1,943,232	1,204,229
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	4,536	297
Fair value measurements on a recurring basis | Level 2 | Bank | Forward commitments
Fair value measurements on a recurring and nonrecurring basis
Derivative assets	0	3
Derivative liabilities	0	12
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	1,849,559	1,026,385
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	62,322	117,787
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	31,351	60,057
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	4,530	2,173
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	27,185	28,597
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
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	27,185	28,597
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