Investment Securities - Amortized cost and carrying value of other investment securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Other investment securities
Carrying Value	$ 49,124,000	$ 19,229,000	$ 49,124,000	$ 19,229,000	$ 25,604,000
Amortized Cost
Due in one year or less	6,012,000		6,012,000
Due after one year through five years	71,916,000		71,916,000
Due after five years through ten years	411,641,000		411,641,000
Due after ten years	1,299,940,000		1,299,940,000
Fair Value	1,789,509,000	1,598,898,000	1,789,509,000	1,598,898,000	1,540,650,000
Fair Value
Due in one year or less	6,018,000		6,018,000
Due after one year through five years	72,679,000		72,679,000
Due after five years through ten years	418,641,000		418,641,000
Due after ten years	1,315,796,000		1,315,796,000
Fair Value	1,813,134,000	1,551,281,000	1,813,134,000	1,551,281,000	1,517,067,000
Information with respect to sales of available-for-sale securities
Sale proceeds			231,990,000	67,853,000
Net realized gain	437,000		2,700,000
Gross realized losses		(11,000)		(652,000)
Amortized Cost
Total		500,000		500,000
Fair Value
Fair Value	0	500,000	0	500,000	0
Investment in Federal Home Loan Bank Stock
Other investment securities
Carrying Value	43,044,000	13,149,000	43,044,000	13,149,000	19,524,000
Investment in unconsolidated subsidiaries
Other investment securities
Carrying Value	3,563,000	3,563,000	3,563,000	3,563,000	3,563,000
Other nonmarketable investment securities
Other investment securities
Carrying Value	$ 2,517,000	$ 2,517,000	$ 2,517,000	$ 2,517,000	$ 2,517,000