Accumulated Other Comprehensive Income, Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2023	Sep. 30, 2023
Changes in Accumulated Other Comprehensive Income by Component [Roll Forward]
Balance attributable to SMP			$ 610,020
Other comprehensive income before reclassifications		$ (1,803)	3,516
Amounts reclassified from accumulated other comprehensive income		(489)	(1,265)
Other comprehensive income, net		(2,292)	2,251
Balance attributable to SMP		629,348	629,348
Reclassification out of Accumulated Other Comprehensive Income [Member]
Changes in Accumulated Other Comprehensive Income by Component [Roll Forward]
Unrecognized gain relating to change in fair value of cash flow interest rate hedge		2,300	3,000
Unrecognized gain relating to change in fair value of cash flow interest rate hedge, net of tax		1,700	2,200
Unrecognized gain, cash receipts		600	1,700
Unrecognized gain, cash receipts, net of tax		500	1,200
Accumulated Other Comprehensive Income (Loss) [Member]
Changes in Accumulated Other Comprehensive Income by Component [Roll Forward]
Balance attributable to SMP		(7,927)	(12,470)
Balance attributable to SMP		(10,219)	(10,219)
Foreign Currency Translation [Member]
Changes in Accumulated Other Comprehensive Income by Component [Roll Forward]
Balance attributable to SMP		(12,234)	(16,330)
Other comprehensive income before reclassifications		(3,997)	99
Amounts reclassified from accumulated other comprehensive income		0	0
Other comprehensive income, net		(3,997)	99
Balance attributable to SMP		(16,231)	(16,231)
Unrecognized Postretirement Benefit Costs (Credit) [Member]
Changes in Accumulated Other Comprehensive Income by Component [Roll Forward]
Balance attributable to SMP		30	37
Other comprehensive income before reclassifications		0	0
Amounts reclassified from accumulated other comprehensive income		(3)	(10)
Other comprehensive income, net		(3)	(10)
Balance attributable to SMP		27	27
Unrealized Derivative Gains (Losses) [Member]
Changes in Accumulated Other Comprehensive Income by Component [Roll Forward]
Balance attributable to SMP		4,277	3,823
Other comprehensive income before reclassifications	[1]	2,194	3,417
Amounts reclassified from accumulated other comprehensive income		(486)	(1,255)
Other comprehensive income, net		1,708	2,162
Balance attributable to SMP		$ 5,985	$ 5,985

[1] Consists of the unrecognized gain relating to the change in fair value of the cash flow interest rate hedge of $2.3 million ($1.7 million, net of tax) and $3 million ($2.2 million, net of tax) in the three months and nine months ended September 30, 2023, respectively, plus cash receipts of $0.6 million ($0.5 million, net of tax) and $1.7 million ($1.2 million, net of tax) in the three months and nine months ended September 30, 2023, respectively.