Fair Value - Financial Assets and Liabilities Measured at Fair Value Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 1,723,445	$ 1,811,813
Loans held for sale	172,683	153,799
Mortgage servicing rights	86,842	95,596	$ 94,850	$ 84,269
U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	28,008	30,335
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	50,954	50,676
Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,723,445	1,811,813
Loans held for sale	172,683	153,799
Mortgage servicing rights	86,842	95,596
Other assets - derivatives	13,820	12,347
Other liabilities - derivatives	3,418	4,213
Recurring Basis [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	28,008	30,335
Recurring Basis [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	50,954	50,676
Recurring Basis [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,644,483	1,730,802
Level 1 [Member] | Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Loans held for sale	0	0
Mortgage servicing rights	0	0
Other assets - derivatives	3,702	5,006
Other liabilities - derivatives	262	66
Level 1 [Member] | Recurring Basis [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Level 1 [Member] | Recurring Basis [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Level 1 [Member] | Recurring Basis [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Level 2 [Member] | Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,723,445	1,811,813
Loans held for sale	172,683	153,799
Mortgage servicing rights	0	0
Other assets - derivatives	8,353	6,154
Other liabilities - derivatives	3,156	4,147
Level 2 [Member] | Recurring Basis [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	28,008	30,335
Level 2 [Member] | Recurring Basis [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	50,954	50,676
Level 2 [Member] | Recurring Basis [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,644,483	1,730,802
Level 3 [Member] | Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Loans held for sale	0	0
Mortgage servicing rights	86,842	95,596
Other assets - derivatives	1,765	1,187
Other liabilities - derivatives	0	0
Level 3 [Member] | Recurring Basis [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Level 3 [Member] | Recurring Basis [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Level 3 [Member] | Recurring Basis [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 0	$ 0