Credit Agreement (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 650,000
Option to increase maximum borrowing capacity		200,000
United States of America, Dollars
Line of Credit Facility [Line Items]
Senior unsecured notes, issued amount		$ 100,000
Debt Instrument, Interest Rate During Period		3.195%
Sweden, Kronor
Line of Credit Facility [Line Items]
Senior unsecured notes, issued amount		$ 150,000
Proceeds from Debt, Net of Issuance Costs		$ 150,000
Debt Instrument, Interest Rate During Period		1.348%
Minimum
Line of Credit Facility [Line Items]
Commitment fee rate		0.125%
Maximum
Line of Credit Facility [Line Items]
Commitment fee rate		0.20%
Bank Of America February Two Thousand Eleven New Credit Agreement
Line of Credit Facility [Line Items]
Term of agreement		5 years
Commitment fee rate		0.175%
Line of Credit Facility, Fair Value of Amount Outstanding		$ 10,800
Term loan facility [Member]
Line of Credit Facility [Line Items]
Proceeds from Debt, Net of Issuance Costs	$ 100,000
Term loan facility [Member] | United States of America, Dollars
Line of Credit Facility [Line Items]
Debt Instrument, Annual Amortization, Percent		5.00%
Proceeds from Debt, Net of Issuance Costs		$ 100,000
Term loan facility [Member] | Sweden, Kronor
Line of Credit Facility [Line Items]
Debt Instrument, Annual Amortization, Percent	5.00%	5.00%
Proceeds from Debt, Net of Issuance Costs	$ 150,000
Credit Agreement
Line of Credit Facility [Line Items]
Term of agreement		5 years
Long-term Debt, Gross		$ 239,552	$ 0
Standby Letters of Credit [Member]
Line of Credit Facility [Line Items]
Letter of Credit		237,200
Revolving Credit Facility
Line of Credit Facility [Line Items]
Proceeds from Debt, Net of Issuance Costs		100,000
Repayments of Lines of Credit		$ 375,000
LIBOR | Bank Of America February Two Thousand Eleven New Credit Agreement
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate at Period End		3.049%
Prime Rate | Maximum
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.375%
Prime Rate | Bank Of America February Two Thousand Eleven New Credit Agreement | Minimum
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.00%