Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale
Amortized Cost	$ 1,866,151	$ 2,422,802
Gross Unrealized Gains	27,959	24,960
Gross Unrealized Losses	(7,593)	(22,285)
Fair Value	1,886,517	2,425,477
Held-to-maturity
Amortized Cost	655,128	675,674
Gross Unrealized Gains	23,503	21,400
Gross Unrealized Losses	(7,433)	(7,985)
Fair Value	671,198	689,089
Total
Amortized Cost	2,521,279	3,098,476
Gross Unrealized Gains	51,462	46,360
Gross Unrealized Losses	(15,026)	(30,270)
Fair Value	2,557,715	3,114,566
U.S. government and federal agency
Available-for-sale
Amortized Cost	34,161	39,554
Gross Unrealized Gains	45	15
Gross Unrealized Losses	(160)	(162)
Fair Value	34,046	39,407
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	19,178	19,557
Gross Unrealized Gains	100	55
Gross Unrealized Losses	(11)	(42)
Fair Value	19,267	19,570
State and local governments
Available-for-sale
Amortized Cost	623,296	775,395
Gross Unrealized Gains	22,616	20,941
Gross Unrealized Losses	(3,939)	(9,963)
Fair Value	641,973	786,373
Held-to-maturity
Amortized Cost	655,128	675,674
Gross Unrealized Gains	23,503	21,400
Gross Unrealized Losses	(7,433)	(7,985)
Fair Value	671,198	689,089
Corporate bonds
Available-for-sale
Amortized Cost	248,865	471,569
Gross Unrealized Gains	1,281	1,175
Gross Unrealized Losses	(156)	(793)
Fair Value	249,990	471,951
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	837,808	1,014,518
Gross Unrealized Gains	3,703	2,744
Gross Unrealized Losses	(2,582)	(9,747)
Fair Value	838,929	1,007,515
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	102,843	102,209
Gross Unrealized Gains	214	30
Gross Unrealized Losses	(745)	(1,578)
Fair Value	$ 102,312	$ 100,661