Debt Securities Available for Sale and Debt Securities Held to Maturity - Gross Unrealized Losses and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 Months, Fair Value	$ 106,329	$ 510,057
Less than 12 Months, Unrealized Losses	184	3,172
12 Months or More, Fair Value	1,365,299	636,101
12 Months or More, Unrealized Losses	36,023	14,439
Total, Fair Value	1,471,628	1,146,158
Total, Unrealized Losses	36,207	17,611
Gross unrealized losses and fair value of debt securities held to maturity that are in a loss position
Less than 12 months, Fair Value	3,074	13,309
Less than 12 months, Unrealized Losses	14	132
12 months or more, Fair Value	31,738	11,470
12 months or more, Unrealized Losses	1,031	280
Total, fair value	34,812	24,779
Total, Unrealized Losses	1,045	412
Mortgage-backed securities, U.S. Government sponsored enterprises and federal agencies
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 Months, Fair Value	102,709	406,298
Less than 12 Months, Unrealized Losses	184	2,686
12 Months or More, Fair Value	838,482	428,585
12 Months or More, Unrealized Losses	26,544	11,126
Total, Fair Value	941,191	834,883
Total, Unrealized Losses	26,728	13,812
Gross unrealized losses and fair value of debt securities held to maturity that are in a loss position
Less than 12 months, Fair Value	3,074	13,309
Less than 12 months, Unrealized Losses	14	132
12 months or more, Fair Value	31,738	11,470
12 months or more, Unrealized Losses	1,031	280
Total, fair value	34,812	24,779
Total, Unrealized Losses	1,045	412
Obligations of states and political subdivisions
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 Months, Fair Value	3,620	103,759
Less than 12 Months, Unrealized Losses	0	486
12 Months or More, Fair Value	526,817	207,516
12 Months or More, Unrealized Losses	9,479	3,313
Total, Fair Value	530,437	311,275
Total, Unrealized Losses	$ 9,479	$ 3,799