Investment Securities - Amortized cost and carrying value of other investment securities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Other investment securities
Carrying Value	$ 49,124	$ 19,229	$ 25,604
Amortized Cost
Due in one year or less	11,037
Due after one year through five years	52,053
Due after five years through ten years	402,345
Due after ten years	1,235,246
Fair Value	1,700,681	1,614,571	1,540,650
Fair Value
Due in one year or less	11,038
Due after one year through five years	52,764
Due after five years through ten years	408,027
Due after ten years	1,245,447
Fair Value	1,717,276	1,577,999	1,517,067
Information with respect to sales of available-for-sale securities
Sale proceeds	189,327	51,822
Amortized Cost
Total		499
Fair Value
Fair Value	0	503	0
Investment in Federal Home Loan Bank Stock
Other investment securities
Carrying Value	43,044	13,149	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