Investment Securities - Amortized cost and carrying value of other investment securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Other investment securities
Carrying Value	$ 40,624	$ 23,479	$ 25,604
Amortized Cost
Due in one year or less	9,437
Due after one year through five years	74,473
Due after five years through ten years	361,457
Due after ten years	1,027,171
Fair Value	1,472,538	1,669,833	1,540,650
Fair Value
Due in one year or less	9,415
Due after one year through five years	74,860
Due after five years through ten years	360,630
Due after ten years	1,021,344
Fair Value	1,466,249	1,640,837	1,517,067
Information with respect to sales of available-for-sale securities
Sale proceeds	134,486	5,215
Amortized Cost
Total		1,274
Fair Value
Fair Value	0		1,284
Investment in Federal Home Loan Bank Stock
Other investment securities
Carrying Value	34,544	17,399	19,524
Investment in unconsolidated subsidiaries
Other investment securities
Carrying Value	3,563	3,563	3,563
Other nonmarketable investment securities
Other investment securities
Carrying Value	$ 2,517	$ 2,517	$ 2,517