Securities (Securities available for sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,124,232	$ 1,893,947
Gross Unrealized Gains	9,320	9,547
Gross Unrealized Losses	(40,167)	(57,630)
Estimated Fair Value	2,093,385	1,845,864
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,107,979	1,879,496
Gross Unrealized Gains	8,977	9,478
Gross Unrealized Losses	(40,167)	(57,630)
Estimated Fair Value	2,076,789	1,831,344
U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,435	4,716
Gross Unrealized Gains	14	16
Gross Unrealized Losses	(34)	(32)
Estimated Fair Value	7,415	4,700
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,593,677	1,321,760
Gross Unrealized Gains	4,656	7,026
Gross Unrealized Losses	(32,933)	(38,286)
Estimated Fair Value	1,565,400	1,290,500
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	506,867	553,020
Gross Unrealized Gains	4,307	2,436
Gross Unrealized Losses	(7,200)	(19,312)
Estimated Fair Value	503,974	536,144
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,253	14,451
Gross Unrealized Gains	343	69
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 16,596	$ 14,520