Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 6	$ 225
Available-for-sale Securities, Gross Realized Losses	0	154
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,763,019		1,708,494
Available-for-sale Securities, Gross Unrealized Gain1	17,721		12,426
Available-for-sale Securities, Gross Unrealized Loss1	(3,683)		(8,996)
Available-for-Sale, Estimated Fair Value	1,777,057		1,711,924
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	563,847us-gaap_HeldToMaturitySecurities		575,186us-gaap_HeldToMaturitySecurities
Held-to-maturity Securities, Gross Unrealized Gain1	15,526		11,566
Held-to-maturity Securities, Gross Unrealized Loss1	(2,442)		(2,219)
Held-to-Maturity, Estimated Fair Value	576,931		584,533
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	769,260		725,408
Available-for-sale Securities, Gross Unrealized Gain1	2,015		895
Available-for-sale Securities, Gross Unrealized Loss1	(1,820)		(5,370)
Available-for-Sale, Estimated Fair Value	769,455		720,933
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	993,459		982,764
Available-for-sale Securities, Gross Unrealized Gain1	15,702		11,526
Available-for-sale Securities, Gross Unrealized Loss1	(1,861)		(3,624)
Available-for-Sale, Estimated Fair Value	1,007,300		990,666
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	335,474		353,176
Held-to-maturity Securities, Gross Unrealized Gain1	8,762		5,563
Held-to-maturity Securities, Gross Unrealized Loss1	(2,235)		(1,758)
Held-to-Maturity, Estimated Fair Value	342,001		356,981
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	300		322
Available-for-sale Securities, Gross Unrealized Gain1	4		5
Available-for-sale Securities, Gross Unrealized Loss1	(2)		(2)
Available-for-Sale, Estimated Fair Value	302		325
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	188,386		188,941
Held-to-maturity Securities, Gross Unrealized Gain1	6,613		5,949
Held-to-maturity Securities, Gross Unrealized Loss1	(190)		(386)
Held-to-Maturity, Estimated Fair Value	194,809		194,504
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	39,531		32,565
Held-to-maturity Securities, Gross Unrealized Gain1	150		54
Held-to-maturity Securities, Gross Unrealized Loss1	(17)		(75)
Held-to-Maturity, Estimated Fair Value	39,664		32,544
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	456		504
Held-to-maturity Securities, Gross Unrealized Gain1	1		0
Held-to-maturity Securities, Gross Unrealized Loss1	0		0
Held-to-Maturity, Estimated Fair Value	$ 457		$ 504

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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