Loans (Not Covered by Share Agreements (Past Due Financing Receivables)) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 31,928	$ 16,650
Current	8,265,021	7,322,288
Nonaccrual	76,548	71,943
Total	8,373,497	7,410,881
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	25,322	15,363
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,501	561
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,105	726
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	20,253	8,324
Current	6,733,309	5,685,784
Nonaccrual	36,896	37,604
Total	6,790,458	5,731,712
Commercial | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	9,700	2,779
Current	2,384,637	1,991,525
Nonaccrual	25,572	25,927
Total	2,419,909	2,020,231
Commercial | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,553	5,545
Current	4,348,672	3,694,259
Nonaccrual	11,324	11,677
Total	4,370,549	3,711,481
Commercial | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14,532	7,393
Commercial | 30-59 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,075	2,574
Commercial | 30-59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	9,457	4,819
Commercial | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,738	205
Commercial | 60-89 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	726	205
Commercial | 60-89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,012	0
Commercial | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,983	726
Commercial | 90 Days or More Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,899	0
Commercial | 90 Days or More Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	84	726
Agricultural and agricultural real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,969	99
Current	504,419	549,376
Nonaccrual	24,676	15,933
Total	533,064	565,408
Agricultural and agricultural real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,734	99
Agricultural and agricultural real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	209	0
Agricultural and agricultural real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	26	0
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,550	5,196
Current	582,257	655,329
Nonaccrual	10,935	13,078
Total	597,742	673,603
Residential real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,382	5,147
Residential real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	72	49
Residential real estate | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	96	0
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,156	3,031
Current	445,036	431,799
Nonaccrual	4,041	5,328
Total	452,233	440,158
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,674	2,724
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	482	307
Consumer | 90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0	$ 0