Loans (Not Covered by Share Agreements (Past Due Financing Receivables)) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 16,650	$ 18,137
Current	7,322,288	6,312,508
Nonaccrual	71,943	62,581
Total	7,410,881	6,393,226
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	15,363	13,375
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	561	3,932
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	726	830
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,324	8,700
Current	5,685,784	4,777,349
Nonaccrual	37,604	23,826
Total	5,731,712	4,809,875
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,779	1,605
Current	1,991,525	1,637,773
Nonaccrual	25,927	7,228
Total	2,020,231	1,646,606
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,545	7,095
Current	3,694,259	3,139,576
Nonaccrual	11,677	16,598
Total	3,711,481	3,163,269
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7,393	6,015
Commercial | 30-59 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,574	1,246
Commercial | 30-59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,819	4,769
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	205	2,585
Commercial | 60-89 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	205	259
Commercial | 60-89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	2,326
Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	726	100
Commercial | 90 Days or More Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	100
Commercial | 90 Days or More Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	726	0
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	99	738
Current	549,376	497,546
Nonaccrual	15,933	13,304
Total	565,408	511,588
Agricultural and agricultural real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	99	604
Agricultural and agricultural real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	134
Agricultural and agricultural real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,196	2,292
Current	655,329	601,120
Nonaccrual	13,078	20,867
Total	673,603	624,279
Residential real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,147	2,022
Residential real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	49	270
Residential real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,031	6,407
Current	431,799	436,493
Nonaccrual	5,328	4,584
Total	440,158	447,484
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,724	4,734
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	307	943
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0	$ 730