Investment Securities (Summary of Investment Securities with Gross Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of continuous unrealized loss position of securities
Less Than 12 Months, Fair Value	$ 1,379,705	$ 1,478,240
Less Than 12 Months, Gross Unrealized Losses	21,534	39,116
12 Months or More, Fair Value	563,188	56,897
12 Months or More, Gross Unrealized Losses	13,649	1,751
Total, Fair Value	1,942,893	1,535,137
Total, Gross Unrealized Losses	35,183	40,867
Government sponsored agencies
Summary of continuous unrealized loss position of securities
Less Than 12 Months, Fair Value	63,818	105,702
Less Than 12 Months, Gross Unrealized Losses	510	1,707
12 Months or More, Fair Value	24,621	15,023
12 Months or More, Gross Unrealized Losses	438	361
Total, Fair Value	88,439	120,725
Total, Gross Unrealized Losses	948	2,068
State and political subdivisions
Summary of continuous unrealized loss position of securities
Less Than 12 Months, Fair Value	437,407	758,063
Less Than 12 Months, Gross Unrealized Losses	12,268	28,158
12 Months or More, Fair Value	349,242	26,810
12 Months or More, Gross Unrealized Losses	10,093	975
Total, Fair Value	786,649	784,873
Total, Gross Unrealized Losses	22,361	29,133
Residential mortgage-backed securities
Summary of continuous unrealized loss position of securities
Less Than 12 Months, Fair Value	93,508	244,239
Less Than 12 Months, Gross Unrealized Losses	383	3,992
12 Months or More, Fair Value	56,576	0
12 Months or More, Gross Unrealized Losses	1,243	0
Total, Fair Value	150,084	244,239
Total, Gross Unrealized Losses	1,626	3,992
Collateralized mortgage obligations
Summary of continuous unrealized loss position of securities
Less Than 12 Months, Fair Value	713,525	279,001
Less Than 12 Months, Gross Unrealized Losses	7,235	3,778
12 Months or More, Fair Value	73,707	14,754
12 Months or More, Gross Unrealized Losses	1,249	225
Total, Fair Value	787,232	293,755
Total, Gross Unrealized Losses	8,484	4,003
Corporate bonds
Summary of continuous unrealized loss position of securities
Less Than 12 Months, Fair Value	71,447	80,536
Less Than 12 Months, Gross Unrealized Losses	1,138	1,401
12 Months or More, Fair Value	47,878	0
12 Months or More, Gross Unrealized Losses	454	0
Total, Fair Value	119,325	80,536
Total, Gross Unrealized Losses	1,592	1,401
Preferred stock and trust preferred securities
Summary of continuous unrealized loss position of securities
Less Than 12 Months, Fair Value	0	10,699
Less Than 12 Months, Gross Unrealized Losses	0	80
12 Months or More, Fair Value	11,164	310
12 Months or More, Gross Unrealized Losses	172	190
Total, Fair Value	11,164	11,009
Total, Gross Unrealized Losses	$ 172	$ 270