Investment Securities - Amortized cost and carrying value of other investment securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other investment securities
Carrying Value	$ 49,124	$ 25,604
Impairment on other investment securities	0
Amortized Cost
Due in one year or less	7,267
Due after one year through five years	54,662
Due after five years through ten years	442,830
Due after ten years	1,436,004
Fair Value	1,940,763	1,540,650
Fair Value
Due in one year or less	7,315
Due after one year through five years	55,286
Due after five years through ten years	447,167
Due after ten years	1,446,279
Fair Value	1,956,047	1,517,067
Information with respect to sales of available-for-sale securities
Sale proceeds	242,733	73,054	$ 374,938
Gross realized gains	6,030	31	1,832
Gross realized losses	(3,319)	(686)	(411)
Information with respect to sales of Held to Maturity
Proceeds from Sale of Held-to-maturity Securities	0	0	$ 0
Investment in Federal Home Loan Bank Stock
Other investment securities
Carrying Value	43,044	19,524
Investment in unconsolidated subsidiaries
Other investment securities
Carrying Value	3,563	3,563
Other nonmarketable investment securities
Other investment securities
Carrying Value	$ 2,517	$ 2,517