Investment Securities - Amortized cost and fair value of other investment securities - (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Available for Sale Securities
Amortized Cost	$ 23,479	$ 23,047	$ 14,726
Fair Value	23,479	23,047	14,726
Amortized Cost
Due in one year or less	1,173
Due after one year through five years	122,525
Due after five years through ten years	374,833
Due after ten years	1,171,302
Fair Value	1,669,833	1,655,107	1,377,433
Fair Value
Due in one year or less	1,175
Due after one year through five years	121,675
Due after five years through ten years	369,402
Due after ten years	1,148,585
Fair Value	1,640,837	1,648,193	1,379,788
Amortized Cost
Due after five years through ten years	1,274
Total	1,274	2,529	6,095
Fair Value
Due after five years through ten years	1,284
Fair Value	1,284	2,556	6,217
Federal Home Loan Bank stock
Available for Sale Securities
Amortized Cost	17,399	16,967	11,239
Fair Value	17,399	16,967	11,239
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