Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($)		Dec. 31, 2016	Dec. 31, 2015
Available-for-sale
Amortized Cost		$ 2,422,802,000	$ 2,588,013,000
Gross Unrealized Gains		24,960,000	41,065,000
Gross Unrealized Losses		(22,285,000)	(18,318,000)
Fair Value		2,425,477,000	2,610,760,000
Held-to-maturity
Amortized Cost		675,674,000	702,072,000
Gross Unrealized Gains		21,400,000	31,863,000
Gross Unrealized Losses		(7,985,000)	(4,422,000)
Fair Value		689,089,000	729,513,000
Total
Amortized Cost		3,098,476,000	3,290,085,000
Gross Unrealized Gains		46,360,000	72,928,000
Gross Unrealized Losses		(30,270,000)	(22,740,000)
Fair Value		3,114,566,000	3,340,273,000
U.S. government and federal agency
Available-for-sale
Amortized Cost		39,554,000	47,868,000
Gross Unrealized Gains		15,000	15,000
Gross Unrealized Losses		(162,000)	(432,000)
Fair Value		39,407,000	47,451,000
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost		19,557,000	93,230,000
Gross Unrealized Gains		55,000	100,000
Gross Unrealized Losses		(42,000)	(163,000)
Fair Value		19,570,000	93,167,000
State and local governments
Available-for-sale
Amortized Cost		775,395,000	856,738,000
Gross Unrealized Gains		20,941,000	34,159,000
Gross Unrealized Losses		(9,963,000)	(5,878,000)
Fair Value		786,373,000	885,019,000
Held-to-maturity
Amortized Cost		675,674,000	702,072,000
Gross Unrealized Gains		21,400,000	31,863,000
Gross Unrealized Losses		(7,985,000)	(4,422,000)
Fair Value		689,089,000	729,513,000
Corporate bonds
Available-for-sale
Amortized Cost		471,569,000	386,629,000
Gross Unrealized Gains		1,175,000	611,000
Gross Unrealized Losses		(793,000)	(3,077,000)
Fair Value		471,951,000	384,163,000
Mortgage-backed securities
Available-for-sale
Amortized Cost		1,116,727,000	1,203,548,000
Gross Unrealized Gains		2,774,000	6,180,000
Gross Unrealized Losses		(11,325,000)	(8,768,000)
Fair Value	[1]	1,108,176,000	1,200,960,000
Residential mortgage backed securities
Available-for-sale
Fair Value		1,011,252,000	1,200,503,000
Commercial mortgage backed securities
Available-for-sale
Fair Value		$ 96,924,000	$ 457,000

[1] Consists of residential mortgage-backed securities with fair values of $1,011,252,000 and $1,200,503,000 at December 31, 2016 and 2015, respectively, and commercial mortgage-backed securities, with fair values of $96,924,000 and $457,000 at December 31, 2016 and 2015, respectively.