Long-Term Debt (Long-Term Debt Phantom) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)
$1.4 billion unsecured revolving credit facility, LIBOR plus 1.175%, currently 2.64% and a facility fee of 0.20%, due 2020
Long-Term Debt
Long term debt, principal amount	$ 1,400,000,000
Long term debt, current interest rate (as a percent)	2.64%	2.64%
Long term debt, facility fee (as a percent)	0.20%
Margin on floating rate base (as a percent)	1.175%
Long term debt, due date (year)	2020
$1.2 billion unsecured revolving credit facility, LIBOR plus 1.175%, currently 2.58% and a facility fee of 0.20%, due 2022
Long-Term Debt
Long term debt, principal amount	$ 1,200,000,000
Long term debt, current interest rate (as a percent)	2.58%	2.58%
Long term debt, facility fee (as a percent)	0.20%
Margin on floating rate base (as a percent)	1.175%
Long term debt, due date (year)	2022
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Period 1
Long-Term Debt
Long term debt, due date (year)	2018
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Period 2
Long-Term Debt
Long term debt, due date (year)	2020
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Period 3
Long-Term Debt
Long term debt, due date (year)	2022
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Period 4
Long-Term Debt
Long term debt, due date (year)	2027
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Lower Limit
Long-Term Debt
Long term debt, stated interest rate (as a percent)	2.65%	2.65%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Lower Limit | Period 1
Long-Term Debt
Long term debt, stated interest rate (as a percent)	2.65%	2.65%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Lower Limit | Period 2
Long-Term Debt
Long term debt, stated interest rate (as a percent)	2.65%	2.65%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Lower Limit | Period 3
Long-Term Debt
Long term debt, stated interest rate (as a percent)	2.65%	2.65%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Lower Limit | Period 4
Long-Term Debt
Long term debt, stated interest rate (as a percent)	2.65%	2.65%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Upper Limit
Long-Term Debt
Long term debt, stated interest rate (as a percent)	7.50%	7.50%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Upper Limit | Period 1
Long-Term Debt
Long term debt, stated interest rate (as a percent)	7.50%	7.50%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Upper Limit | Period 2
Long-Term Debt
Long term debt, stated interest rate (as a percent)	7.50%	7.50%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Upper Limit | Period 3
Long-Term Debt
Long term debt, stated interest rate (as a percent)	7.50%	7.50%
Unsecured senior notes and senior debentures, 2.65% to 7.50%, due 2018, 2020, 2022, 2027 and 2028 | Upper Limit | Period 4
Long-Term Debt
Long term debt, stated interest rate (as a percent)	7.50%	7.50%
$200 million unsecured term loan, LIBOR plus 1.30%
Long-Term Debt
Long term debt, principal amount	$ 200,000,000
Margin on floating rate base (as a percent)	1.30%
$841.8 million unsecured term loan, LIBOR plus 1.00%, currently 2.52% due through 2028
Long-Term Debt
Long term debt, principal amount	$ 841,800,000
Long term debt, current interest rate (as a percent)	2.52%	2.52%
Margin on floating rate base (as a percent)	1.00%
Long term debt, due date (year)	2028
$226.1 million unsecured term loan, 2.53%, due through 2028
Long-Term Debt
Long term debt, principal amount	$ 226,100,000
Long term debt, current interest rate (as a percent)	2.53%	2.53%
Long term debt, due date (year)	2028
€700.7 million unsecured term loan, EURIBOR plus 1.15% currently 1.15%, due through 2028
Long-Term Debt
Long term debt, principal amount | €		€ 700,700,000
Long term debt, current interest rate (as a percent)	1.15%	1.15%
Margin on floating rate base (as a percent)	1.15%
Long term debt, due date (year)	2028
$742.1 million unsecured term loan, LIBOR plus 1.30%, currently 2.81%, due through 2027
Long-Term Debt
Long term debt, principal amount	$ 742,100,000.0
Long term debt, current interest rate (as a percent)	2.81%	2.81%
Margin on floating rate base (as a percent)	1.30%
Long term debt, due date (year)	2027
$273.2 million unsecured term loan, LIBOR plus 1.75%
Long-Term Debt
Long term debt, principal amount	$ 273,200,000.0
Margin on floating rate base (as a percent)	1.75%
$519 million unsecured term loan, LIBOR plus 0.45%, currently 2.00%, due through 2020
Long-Term Debt
Long term debt, principal amount	$ 519,000,000
Long term debt, current interest rate (as a percent)	2.00%	2.00%
Margin on floating rate base (as a percent)	0.45%
Long term debt, due date (year)	2020
$420 million unsecured term loan, 5.41%, due through 2021
Long-Term Debt
Long term debt, principal amount	$ 420,000,000
Long term debt, stated interest rate (as a percent)	5.41%	5.41%
Long term debt, due date (year)	2021
$420 million unsecured term loan, LIBOR plus 1.65%, currently 3.21%, due through 2021
Long-Term Debt
Long term debt, principal amount	$ 420,000,000
Long term debt, current interest rate (as a percent)	3.21%	3.21%
Margin on floating rate base (as a percent)	1.65%
Long term debt, due date (year)	2021
€159.4 million unsecured term loan, EURIBOR plus 1.58%, currently 1.58%, due through 2021
Long-Term Debt
Long term debt, principal amount | €		€ 159,400,000.0
Long term debt, current interest rate (as a percent)	1.58%	1.58%
Margin on floating rate base (as a percent)	1.58%
Long term debt, due date (year)	2021
$524.5 million unsecured term loan, LIBOR plus 0.50%, currently 1.96%, due through 2021
Long-Term Debt
Long term debt, principal amount	$ 524,500,000.0
Long term debt, current interest rate (as a percent)	1.96%	1.96%
Margin on floating rate base (as a percent)	0.50%
Long term debt, due date (year)	2021
$566.1 million unsecured term loan, LIBOR plus 0.37%, currently 1.90%, due through 2022
Long-Term Debt
Long term debt, principal amount	$ 566,100,000.0
Long term debt, current interest rate (as a percent)	1.90%	1.90%
Margin on floating rate base (as a percent)	0.37%
Long term debt, due date (year)	2022
$1.1 billion unsecured term loan, LIBOR plus 1.65%, currently 3.21%, due through 2022
Long-Term Debt
Long term debt, principal amount	$ 1,100,000,000
Long term debt, current interest rate (as a percent)	3.21%	3.21%
Margin on floating rate base (as a percent)	1.65%
Long term debt, due date (year)	2022
$632.0 million unsecured term loan, LIBOR plus 0.40%, currently 1.86%, due through 2023
Long-Term Debt
Long term debt, principal amount	$ 632,000,000
Long term debt, current interest rate (as a percent)	1.86%	1.86%
Margin on floating rate base (as a percent)	0.40%
Long term debt, due date (year)	2023
$673.5 million unsecured term loan, LIBOR plus 0.40%, currently 1.92%, due through 2024
Long-Term Debt
Long term debt, principal amount	$ 673,500,000.0
Long term debt, current interest rate (as a percent)	1.92%	1.92%
Margin on floating rate base (as a percent)	0.40%
Long term debt, due date (year)	2024
$65.0 million unsecured term loan, LIBOR plus 1.45%, currently 3.02%, due through 2019
Long-Term Debt
Long term debt, principal amount	$ 65,000,000
Long term debt, current interest rate (as a percent)	3.02%	3.02%
Margin on floating rate base (as a percent)	1.45%
Long term debt, due date (year)	2019
$380.0 million unsecured term loan, LIBOR plus 1.45%, currently 3.02%, due 2018
Long-Term Debt
Long term debt, principal amount	$ 380,000,000
Long term debt, current interest rate (as a percent)	3.02%	3.02%
Margin on floating rate base (as a percent)	1.45%
Long term debt, due date (year)	2018
$791.1 million unsecured term loan, LIBOR plus 1.30%, currently 2.85%, due through 2026
Long-Term Debt
Long term debt, principal amount	$ 791,100,000.0
Long term debt, current interest rate (as a percent)	2.85%	2.85%
Margin on floating rate base (as a percent)	1.30%
Long term debt, due date (year)	2026
$290.0 million unsecured term loan, LIBOR plus 1.75%
Long-Term Debt
Long term debt, principal amount	$ 290,000,000
Margin on floating rate base (as a percent)	1.75%
€365 million unsecured term loan, EURIBOR plus 1.75%
Long-Term Debt
Long term debt, principal amount | €		€ 365,000,000
Margin on floating rate base (as a percent)	1.75%
$7.3 million unsecured term loan, LIBOR plus 2.5%
Long-Term Debt
Long term debt, principal amount	$ 7,300,000.0
Margin on floating rate base (as a percent)	2.50%
$30.3 million unsecured term loan, LIBOR plus 3.75%, currently 5.29%, due through 2021
Long-Term Debt
Long term debt, principal amount	$ 30,300,000.0
Long term debt, current interest rate (as a percent)	5.29%	5.29%
Margin on floating rate base (as a percent)	3.75%
Long term debt, due date (year)	2021
€80.0 million unsecured term loan, EURIBOR plus 1.32% currently 1.32%, due through 2024
Long-Term Debt
Long term debt, principal amount | €		€ 80,000,000
Long term debt, current interest rate (as a percent)	1.32%	1.32%
Long term debt, facility fee (as a percent)	0.20%
Margin on floating rate base (as a percent)	1.32%
Long term debt, due date (year)	2024