Bank segment - Contractual maturities of investment securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Available-for-sale
Due in one year or less	$ 0		$ 0
Due after one year through five years	79,832		79,832
Due after five years through ten years	45,627		45,627
Due after ten years	15,427		15,427
Total amortized cost	140,886		140,886
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	2,381,062		2,381,062
Amortized cost	2,521,948		2,521,948
Held-to-maturity
Due in one year or less	0		0
Due after one year through five years	0		0
Due after five years through ten years	40,065		40,065
Due after ten years	0		0
Total amortized cost	40,065		40,065
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	335,590		335,590
Amortized cost	375,655		375,655
Available-for-sale
Due in one year or less	0		0
Due after one year through five years	81,989		81,989
Due after five years through ten years	46,189		46,189
Due after ten years	15,427		15,427
Total fair value	143,605		143,605
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	2,366,301		2,366,301
Total available-for-sale securities	2,509,906		2,509,906		$ 1,970,417
Held-to-maturity
Due in one year or less	0		0
Due after one year through five years	0		0
Due after five years through ten years	40,381		40,381
Due after ten years	0		0
Total fair value	40,381		40,381
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies	333,760		333,760
Total held-to-maturity securities	374,141		374,141		$ 229,963
Proceeds, gross gains and losses from sales of available-for-sale securities
Proceeds	0	$ 169,157	197,354	$ 169,157
Gross gains	0	9,312	975	9,312
Gross losses	0	37	447	37
Tax expense on realized gains	$ 0	$ 2,492	$ 142	$ 2,492