Investment Securities - Amortized cost and fair value of other investment securities - (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Information with respect to sales of available-for-sale securities
Proceeds from Sale of Available-for-sale Securities	$ 265,965	$ 137
Amortized Cost
Due in one year or less	12,023
Due after one year through five years	109,509
Due after five years through ten years	275,960
Due after ten years	919,535
Total	1,317,027	909,150	$ 1,002,166
Fair Value
Due in one year or less	12,090
Due after one year through five years	110,127
Due after five years through ten years	278,021
Due after ten years	920,441
Fair Value	1,320,679	925,374	999,405
Amortized Cost
Due in one year or less	2,225
Due after one year through five years	1,153
Due after five years through ten years	300
Total	3,678	6,851	6,094
Fair Value
Due in one year or less	2,253
Due after one year through five years	1,179
Due after five years through ten years	300
Fair Value	3,732	7,076	6,250
Amortized Cost	12,439	9,482	9,482
Fair Value	12,439	9,482	9,482
Federal Home Loan Bank stock
Fair Value
Amortized Cost	10,177	7,840	7,840
Fair Value	10,177	7,840	7,840
Investment in unconsolidated subsidiaries
Fair Value
Amortized Cost	2,262	1,642	1,642
Fair Value	$ 2,262	$ 1,642	$ 1,642