Debt - Trust Investment Grade Rating (Details)	9 Months Ended
	Sep. 30, 2017
Credit rating at least A- or A3 | Adjusted LIBOR rate loans and letter of credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	0.85%
Credit rating at least A- or A3 | Adjusted LIBOR Rate Term Loans And Letter Of Credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.40%
Credit rating at least BBBplus or BAA1 | Adjusted LIBOR rate loans and letter of credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	0.90%
Credit rating at least BBBplus or BAA1 | Adjusted LIBOR Rate Term Loans And Letter Of Credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.45%
Credit rating at least BBB or BAA2 | Adjusted LIBOR rate loans and letter of credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.00%
Credit rating at least BBB or BAA2 | Adjusted LIBOR Rate Term Loans And Letter Of Credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.55%
Credit rating at least BBB- or BAA3 | Adjusted LIBOR rate loans and letter of credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.20%
Credit rating at least BBB- or BAA3 | Adjusted LIBOR Rate Term Loans And Letter Of Credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.80%
Credit rating below BBB- or BAA3 | Adjusted LIBOR rate loans and letter of credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.55%
Credit rating below BBB- or BAA3 | Adjusted LIBOR Rate Term Loans And Letter Of Credit | London Interbank Offered Rate (LIBOR)
Debt
Reference rate (as a percent)	1.25%
Term Loan | Credit rating at least A- or A3 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.40%
Term Loan | Credit rating at least BBBplus or BAA1 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.45%
Term Loan | Credit rating at least BBB or BAA2 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.55%
Term Loan | Credit rating at least BBB- or BAA3 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.80%
Term Loan | Credit rating below BBB- or BAA3 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	1.25%
$850 million unsecured revolving credit facility bearing variable interest of LIBOR plus 1.20%, due September 2020 | Credit rating at least A- or A3 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.00%
$850 million unsecured revolving credit facility bearing variable interest of LIBOR plus 1.20%, due September 2020 | Credit rating at least BBBplus or BAA1 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.00%
$850 million unsecured revolving credit facility bearing variable interest of LIBOR plus 1.20%, due September 2020 | Credit rating at least BBB or BAA2 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.10%
$850 million unsecured revolving credit facility bearing variable interest of LIBOR plus 1.20%, due September 2020 | Credit rating at least BBB- or BAA3 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.20%
$850 million unsecured revolving credit facility bearing variable interest of LIBOR plus 1.20%, due September 2020 | Credit rating below BBB- or BAA3 | Base rate loans | Base rate
Debt
Reference rate (as a percent)	0.60%