Investment Securities - Amortized cost and fair value of other investment securities - (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Available for Sale Securities
Amortized Cost	$ 19,229	$ 23,047	$ 14,301
Fair Value	19,229	23,047	14,301
Amortized Cost
Due in one year or less	2,701
Due after one year through five years	82,771
Due after five years through ten years	391,946
Due after ten years	1,137,153
Fair Value	1,614,571	1,655,107	1,336,379
Fair Value
Due in one year or less	2,703
Due after one year through five years	82,207
Due after five years through ten years	383,572
Due after ten years	1,109,517
Fair Value	1,577,999	1,648,193	1,340,427
Amortized Cost
Due after five years through ten years	499
Total	499	2,529	4,166
Fair Value
Due after five years through ten years	503
Fair Value	503	2,556	4,248
Federal Home Loan Bank stock
Available for Sale Securities
Amortized Cost	13,149	16,967	10,814
Fair Value	13,149	16,967	10,814
Investment in unconsolidated subsidiaries
Available for Sale Securities
Amortized Cost	3,563	3,563	2,262
Fair Value	3,563	3,563	2,262
Other nonmarketable investment securities
Available for Sale Securities
Amortized Cost	2,517	2,517	1,225
Fair Value	$ 2,517	$ 2,517	$ 1,225