Short-Term Borrowings (Details) - USD ($)	1 Months Ended	3 Months Ended	9 Months Ended
	Feb. 28, 2021	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Ratio of Indebtedness to Net Capital		0.65	0.65
Notional Amount of Nonderivative Instruments	$ 40,000,000	$ 70,000,000	$ 30,000,000
Line of Credit Facility, Remaining Borrowing Capacity		203,400,000	203,400,000
Short-term borrowing		$ 192,026,000	$ 192,026,000	$ 175,644,000
Short-term Debt, Weighted Average Interest Rate, at Point in Time		0.82%	0.82%	1.28%
Draws on letters of credit			$ 4,600,000
Financing Receivable, Revolving		$ 400,000,000	400,000,000
Revolving Line of Credit, Short-term
Short-term Debt [Line Items]
Financing Receivable, Revolving		$ 200,000,000	$ 200,000,000
Debt Instrument, Interest Rate, Stated Percentage		70.00%	70.00%
Revolving Line of Credit, Long-term
Short-term Debt [Line Items]
Financing Receivable, Revolving		$ 200,000,000	$ 200,000,000
Debt Instrument, Fee			0.09
Debt Instrument, Interest Rate, Stated Percentage		95.00%	95.00%
3.48% note, due May 31, 2038 [Member]
Short-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.48%	3.48%
Committed Line of Credit Facility Two [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 0.75 percent
Committed Line of Credit Facility Three [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			Lender's base rate, plus 0.75 percent
Committed Line of Credit Facility Five [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			Lender's base rate, plus 0.85 percent
Committed Line of Credit Facility Four [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 1.125 percent
Committed Line of Credit Facility Six [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 1.75 percent
Committed Line of Credit Facility Seven [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 1.75 percent
Committed Line of Credit Facility Eight [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 1.75 percent
Committed Line of Credit Facility Nine [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 1.75 percent
Committed Line of Credit Facility One [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis			LIBOR rate, plus 0.75 percent
Revolving Credit Facility
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 400,000,000	$ 400,000,000
Interest Rate Swap Rate, Low Range [Member]
Short-term Debt [Line Items]
Fixed Swap Rate				0.20%
Interest Rate Swap Rate, High Range [Member]
Short-term Debt [Line Items]
Fixed Swap Rate		0.17%	0.17%	0.205%