Bank segment - Available-for-sale securities, narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Bank Subsidiary [Abstract]
Proceeds from sale of available-for-sale investment securities	$ 169,200,000	$ 0	$ 169,157,000	$ 0
Gross realized gains on available-for-sale securities	9,300,000	0	9,300,000	0
Gross realized losses on available-for-sale securities	0	$ 0	0	$ 0
Tax expense on realized gains	$ 2,500,000		$ 2,500,000