Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Jun. 30, 2019
Securities available-for-sale [Abstract]
Amortized Cost		$ 3,131,227	$ 3,192,462
Gross Unrealized Gains		7,934	74,865
Gross Unrealized Losses		55,475	10,421
Securities available-for-sale, at fair value		3,083,686	3,256,906
Available-for-sale, Amortized Cost [Abstract]
Due in one year or less			64,432
Due in one year to five years			18,158
Due in five years to ten years			85,576
Due after ten years			1,565,650
Mortgage-backed securities			1,289,189
Asset-backed securities			169,457
Amortized Cost			3,192,462
Available-for-sale, Fair Value [Abstract]
Due in one year or less			64,479
Due in one year to five years			18,203
Due in five years to ten years			85,416
Due after ten years			1,627,472
Mortgage-backed securities			1,291,913
Asset-backed securities			169,423
Securities available-for-sale, at fair value		3,083,686	3,256,906
Schedule of Held-to-maturity Securities [Line Items]
Available-for-sale securities transferred to Held-to-Maturity		179,800
Net unrealized after tax loss	$ 2,200
Securities held-to-maturity [Abstract]
Amortized Cost		194,282	190,928
Gross Unrealized Gains		152	9,639
Gross Unrealized Losses		1,303	1
Securities held-to-maturity, fair value		193,131	200,566
Held-to-maturity, Amortized Cost [Abstract]
Due in one year or less			1,401
Due in one year to five years			1,020
Due in five years to ten years			5,779
Due after ten years			182,728
Mortgage-backed securities			0
Asset-backed securities			0
Amortized Cost		194,282	190,928
Held-to-maturity, Fair Value [Abstract]
Due in one year or less			1,401
Due in one year to five years			1,022
Due in five years to ten years			5,866
Due after ten years			192,277
Mortgage-backed securities			0
Asset-backed securities			0
Fair Value		193,131	200,566
U.S. Treasury securities
Securities available-for-sale [Abstract]
Amortized Cost		30,325	48,883
Gross Unrealized Gains		0	34
Gross Unrealized Losses		25	0
Securities available-for-sale, at fair value		30,300	48,917
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		30,300	48,917
U.S. government agency securities
Securities available-for-sale [Abstract]
Amortized Cost		71,456	55,907
Gross Unrealized Gains		49	111
Gross Unrealized Losses		1,346	995
Securities available-for-sale, at fair value		70,159	55,023
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		70,159	55,023
Mortgage-backed securities
Securities available-for-sale [Abstract]
Amortized Cost		1,336,469	1,289,189
Gross Unrealized Gains		3,110	9,155
Gross Unrealized Losses		28,634	6,431
Securities available-for-sale, at fair value		1,310,945	1,291,913
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		1,310,945	1,291,913
State and municipal securities
Securities available-for-sale [Abstract]
Amortized Cost		1,244,471	1,572,847
Gross Unrealized Gains		3,785	64,161
Gross Unrealized Losses		18,602	506
Securities available-for-sale, at fair value		1,229,654	1,636,502
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		1,229,654	1,636,502
Asset-backed securities
Securities available-for-sale [Abstract]
Amortized Cost		379,107	169,457
Gross Unrealized Gains		820	942
Gross Unrealized Losses		4,345	976
Securities available-for-sale, at fair value		375,582	169,423
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		375,582	169,423
Corporate notes and other
Securities available-for-sale [Abstract]
Amortized Cost		69,399	56,179
Gross Unrealized Gains		170	462
Gross Unrealized Losses		2,523	1,513
Securities available-for-sale, at fair value		67,046	55,128
Available-for-sale, Fair Value [Abstract]
Securities available-for-sale, at fair value		67,046	55,128
State and municipal securities
Securities held-to-maturity [Abstract]
Amortized Cost		194,282	190,928
Gross Unrealized Gains		152	9,639
Gross Unrealized Losses		1,303	1
Securities held-to-maturity, fair value		193,131	200,566
Held-to-maturity, Amortized Cost [Abstract]
Amortized Cost		194,282	190,928
Held-to-maturity, Fair Value [Abstract]
Fair Value		$ 193,131	$ 200,566