Debt Debt (Schedule of Senior Note Borrowings) (Details) - Senior Notes € in Thousands	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Mar. 14, 2019 USD ($)	Dec. 31, 2015 EUR (€)	Dec. 19, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jul. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
2023 Series A Notes
Debt Instrument [Line Items]
Principal Amount	$ 75,000,000							$ 75,000,000
Coupon Rate	3.94%	3.94%						3.94%
2025 Series B Notes
Debt Instrument [Line Items]
Principal Amount	$ 75,000,000							$ 75,000,000
Coupon Rate	4.04%	4.04%						4.04%
2026 Senior Notes
Debt Instrument [Line Items]
Principal Amount	$ 75,000,000					$ 75,000,000
Coupon Rate	3.72%	3.72%				3.72%
2021 Series A Notes
Debt Instrument [Line Items]
Principal Amount	$ 50,000,000						$ 50,000,000
Coupon Rate	3.32%	3.32%					3.32%
2024 Series B Notes
Debt Instrument [Line Items]
Principal Amount	$ 75,000,000						$ 75,000,000
Coupon Rate	3.76%	3.76%					3.76%
2025 Series C Notes
Debt Instrument [Line Items]
Principal Amount | €		€ 88,857		€ 88,900
Coupon Rate	1.785%	1.785%		1.785%
2022 Series A Notes
Debt Instrument [Line Items]
Principal Amount	$ 75,000,000				$ 75,000,000
Coupon Rate	3.25%	3.25%			3.25%
2027 Series B Notes
Debt Instrument [Line Items]
Principal Amount	$ 75,000,000				$ 75,000,000
Coupon Rate	3.72%	3.72%			3.72%
2029 Series C Notes
Debt Instrument [Line Items]
Principal Amount	$ 100,000,000		$ 100,000,000
Coupon Rate	4.19%	4.19%	4.19%