Commitments and Contingent Liabilities (Schedule of Commitment Expiration) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Guarantor Obligations [Line Items]
Total Amounts Committed	$ 688,113
Less Than 1 Year	646,026
1-3 Years	33,413
3-5 Years	7,894
Beyond 5 Years	780
Letters of Credit
Guarantor Obligations [Line Items]
Total Amounts Committed	88,111
Less Than 1 Year	70,111
1-3 Years	18,000
3-5 Years	0
Beyond 5 Years	0
Letters of Credit | Employee-Related
Guarantor Obligations [Line Items]
Total Amounts Committed	77,266
Less Than 1 Year	60,364
1-3 Years	16,902
3-5 Years	0
Beyond 5 Years	0
Letters of Credit | Environmental
Guarantor Obligations [Line Items]
Total Amounts Committed	998
Less Than 1 Year	600
1-3 Years	398
3-5 Years	0
Beyond 5 Years	0
Letters of Credit | Other
Guarantor Obligations [Line Items]
Total Amounts Committed	9,847
Less Than 1 Year	9,147
1-3 Years	700
3-5 Years	0
Beyond 5 Years	0
Surety Bonds
Guarantor Obligations [Line Items]
Total Amounts Committed	566,700
Less Than 1 Year	566,699
1-3 Years	0
3-5 Years	1
Beyond 5 Years	0
Surety Bonds | Employee-Related
Guarantor Obligations [Line Items]
Total Amounts Committed	108,948
Less Than 1 Year	108,948
1-3 Years	0
3-5 Years	0
Beyond 5 Years	0
Surety Bonds | Environmental
Guarantor Obligations [Line Items]
Total Amounts Committed	453,035
Less Than 1 Year	453,035
1-3 Years	0
3-5 Years	0
Beyond 5 Years	0
Surety Bonds | Other
Guarantor Obligations [Line Items]
Total Amounts Committed	4,717
Less Than 1 Year	4,716
1-3 Years	0
3-5 Years	1
Beyond 5 Years	0
Guarantees
Guarantor Obligations [Line Items]
Total Amounts Committed	33,302
Less Than 1 Year	9,216
1-3 Years	15,413
3-5 Years	7,893
Beyond 5 Years	780
Guarantees | Other
Guarantor Obligations [Line Items]
Total Amounts Committed	33,302
Less Than 1 Year	9,216
1-3 Years	15,413
3-5 Years	7,893
Beyond 5 Years	$ 780