Investment Securities - Amortized cost and fair value of other investment securities - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Information with respect to sales of available-for-sale securities
Proceeds from Sale of Available-for-sale Securities	$ 374,938	$ 137
Gross realized gains	1,832	122
Gross realized losses	(411)
Net realized gain	1,421	122
Amortized Cost
Due in one year or less	3,143
Due after one year through five years	101,866
Due after five years through ten years	369,235
Due after ten years	1,183,380
Total	1,657,624	1,002,166
Fair Value
Due in one year or less	3,143
Due after one year through five years	101,509
Due after five years through ten years	369,038
Due after ten years	1,177,020
Fair Value	1,650,710	999,405
Amortized Cost
Due after one year through five years	655
Due after five years through ten years	1,874
Total	2,529	6,094
Fair Value
Due after one year through five years	661
Due after five years through ten years	1,895
Fair Value	2,556	6,250
Amortized Cost	20,530	9,482
Fair Value	20,530	9,482
Federal Home Loan Bank stock
Fair Value
Amortized Cost	16,967	7,840
Fair Value	16,967	7,840
Unconsolidated subsidiaries
Fair Value
Amortized Cost	3,563	1,642
Fair Value	$ 3,563	$ 1,642