Fair Value - Financial Assets and Liabilities Measured at Fair Value Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 2,369,089	$ 2,356,682
Loans held for sale	204,157	175,927
Mortgage servicing rights	81,477	80,239	$ 65,514	$ 74,007
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	89,144	115,373
Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,369,089	2,356,682
Loans held for sale	204,157	175,927
Mortgage servicing rights	81,477	80,239
Other assets - derivatives	791	2,518
Other liabilities - derivatives	2,203	412
Recurring Basis [Member] | US Treasury and Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	49,994	56,039
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	89,144	115,373
Recurring Basis [Member] | Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,229,951	2,185,270
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	(2,012)	(524)
Other liabilities - derivatives	1,063	1,174
Level 1 [Member] | Recurring Basis [Member] | US Treasury and Government [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	2,369,089	2,356,682
Loans held for sale	204,157	175,927
Mortgage servicing rights	0	0
Other assets - derivatives	1,797	2,041
Other liabilities - derivatives	1,140	(762)
Level 2 [Member] | Recurring Basis [Member] | US Treasury and Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	49,994	56,039
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	89,144	115,373
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	2,229,951	2,185,270
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	81,477	80,239
Other assets - derivatives	1,006	1,001
Other liabilities - derivatives	0	0
Level 3 [Member] | Recurring Basis [Member] | US Treasury and Government [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