Short-Term and Long-Term Borrowings - Lines of Credit (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Line Limit	$ 340,000,000	$ 340,000,000
Amount Outstanding	97,857,000	80,997,000
Letters of Credit	13,159,000
Amount Available	228,984,000	244,902,000
OTC Credit Agreement
Line of Credit Facility [Line Items]
Line Limit	170,000,000	170,000,000
Amount Outstanding	36,624,000
Letters of Credit	0
Amount Available	133,376,000	104,834,000
OTP Credit Agreement
Line of Credit Facility [Line Items]
Line Limit	170,000,000	170,000,000
Amount Outstanding	61,233,000
Letters of Credit	13,159,000
Amount Available	$ 95,608,000	$ 140,068,000