Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Debt Securities, Held-to-maturity, Transfer, Amount	$ 281,100
Debt Securities, Held-to-maturity, transfer, Fair value amount	275,000
Available-for-Sale:
Amortized Cost	2,579,000	$ 2,305,200
Gross Unrealized Gains	10,700	2,600
Gross Unrealized Losses	(17,700)	(37,100)
Estimated Fair Value	2,572,000	2,270,700
Held-to-Maturity:
Amortized Cost	117,000	406,800
Gross Unrealized Gains	1,500	2,200
Gross Unrealized Losses	(500)	(8,300)
Estimated Fair Value	118,000	400,700
Debt Securities, Held-to-maturity, Transfer, Derivative Hedge, Gain (Loss)	6,000
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	4,000	2,600
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	4,000	2,600
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	601,400	569,300
Gross Unrealized Gains	300	100
Gross Unrealized Losses	(4,000)	(10,200)
Estimated Fair Value	597,700	559,200
Held-to-Maturity:
Amortized Cost	19,800	19,800
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(200)	(300)
Estimated Fair Value	19,600	19,500
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	1,735,900	1,566,400
Gross Unrealized Gains	9,400	2,500
Gross Unrealized Losses	(12,400)	(24,100)
Estimated Fair Value	1,732,900	1,544,800
Held-to-Maturity:
Amortized Cost	1,300	189,700
Gross Unrealized Gains	0	300
Gross Unrealized Losses	0	(6,500)
Estimated Fair Value	1,300	183,500
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	66,200	72,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(900)	(1,800)
Estimated Fair Value	65,300	70,200
State, county and municipal securities
Available-for-Sale:
Amortized Cost	77,700
Gross Unrealized Gains	800
Gross Unrealized Losses	(100)
Estimated Fair Value	78,400
Held-to-Maturity:
Amortized Cost	67,600	150,900
Gross Unrealized Gains	1,500	1,800
Gross Unrealized Losses	(200)	(900)
Estimated Fair Value	68,900	151,800
Corporate securities
Available-for-Sale:
Amortized Cost	92,100	92,900
Gross Unrealized Gains	200	0
Gross Unrealized Losses	(300)	(1,000)
Estimated Fair Value	92,000	91,900
Held-to-Maturity:
Amortized Cost	28,200	46,300
Gross Unrealized Gains	0	100
Gross Unrealized Losses	(100)	(600)
Estimated Fair Value	28,100	45,800
Other investments
Available-for-Sale:
Amortized Cost	1,700	2,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	1,700	2,000
Other Investments
Held-to-Maturity:
Amortized Cost	100	100
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 100	$ 100