Securities (Securities available for sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,680,299	$ 1,584,703
Gross Unrealized Gains	22,838	15,849
Gross Unrealized Losses	(12,621)	(22,118)
Estimated Fair Value	1,690,516	1,578,434
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,667,149	1,571,561
Gross Unrealized Gains	22,611	15,809
Gross Unrealized Losses	(12,621)	(22,074)
Estimated Fair Value	1,677,139	1,565,296
U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,479	25,847
Gross Unrealized Gains	140	22
Gross Unrealized Losses	0	(103)
Estimated Fair Value	6,619	25,766
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,386,707	1,254,452
Gross Unrealized Gains	14,448	9,134
Gross Unrealized Losses	(12,468)	(20,884)
Estimated Fair Value	1,388,687	1,242,702
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	273,147	290,522
Gross Unrealized Gains	8,008	6,547
Gross Unrealized Losses	(152)	(1,087)
Estimated Fair Value	281,003	295,982
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	816	740
Gross Unrealized Gains	15	106
Gross Unrealized Losses	(1)	0
Estimated Fair Value	830	846
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,150	13,142
Gross Unrealized Gains	227	40
Gross Unrealized Losses	0	(44)
Estimated Fair Value	$ 13,377	$ 13,138