Securities - Schedule of Debt Securities Components (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
AVAILABLE FOR SALE
Amortized Cost		$ 2,179,809	$ 2,013,485
Gross unrealized gains		61,982	10,554
Gross unrealized losses		1,410	34,603
Estimated fair value		2,240,381	1,989,436
HELD TO MATURITY
Amortized cost		140,955	162,931
Gross unrealized gains		6,206	360
Gross unrealized losses		121	3,510
Estimated Fair Value		147,040	159,781
State and political subdivisions
AVAILABLE FOR SALE
Amortized Cost		635,299	728,142
Gross unrealized gains		25,716	6,115
Gross unrealized losses		922	17,656
Estimated fair value		660,093	716,601
HELD TO MATURITY
Amortized cost		2,895	3,083
Gross unrealized gains		33	5
Gross unrealized losses		0	42
Estimated Fair Value		2,928	3,046
Other stocks and bonds
AVAILABLE FOR SALE
Amortized Cost		3,000	3,000
Gross unrealized gains		0	0
Gross unrealized losses		44	291
Estimated fair value		2,956	2,709
Residential
AVAILABLE FOR SALE
Amortized Cost	[1]	1,309,776	738,585
Gross unrealized gains	[1]	29,454	3,498
Gross unrealized losses	[1]	441	9,111
Estimated fair value	[1]	1,338,789	732,972
HELD TO MATURITY
Amortized cost	[1]	59,739	59,655
Gross unrealized gains	[1]	3,292	154
Gross unrealized losses	[1]	110	1,140
Estimated Fair Value	[1]	62,921	58,669
Commercial
AVAILABLE FOR SALE
Amortized Cost	[1]	231,734	543,758
Gross unrealized gains	[1]	6,812	941
Gross unrealized losses	[1]	3	7,545
Estimated fair value	[1]	238,543	537,154
HELD TO MATURITY
Amortized cost	[1]	78,321	100,193
Gross unrealized gains	[1]	2,881	201
Gross unrealized losses	[1]	11	2,328
Estimated Fair Value	[1]	$ 81,191	$ 98,066

[1] All mortgage-backed securities issued and/or guaranteed by U.S. government agencies or U.S. government-sponsored enterprises.