Indebtedness (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 20, 2019	Mar. 25, 2019	Mar. 31, 2018	Sep. 30, 2016	May 26, 2016
Debt Instrument [Line Items]
Short-term Debt				$ 70,000,000				$ 11,000,000
Initial Interest Rate	3.93%
Unamortized Debt Issuance Expense	$ 1,100,000
Long-term Debt, Current Maturities	146,328,000
Long-term Debt	2,567,022,000	$ 2,299,625,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	182,732,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	222,774,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	160,902,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	358,597,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,608,496,000
Long-term Debt	2,679,829,000
Interest Expense	111,964,000	95,564,000	$ 67,468,000
Line of Credit Facility, Amount Outstanding	0	57,000,000
Amortization of Financing Costs	$ 6,450,000	7,108,000	5,084,000
Spread over Initial Interest Rate option 1	200
Spread over Initial Interest Rate option 2	400
Indebtedness Weighted Average Remaining Maturity	9 years 5 months
Line of Credit Facility, Maximum Borrowing Capacity							$ 200,000,000
term loan [Member] [Member]
Debt Instrument [Line Items]
Loans Receivable, Basis Spread on Variable Rate	1.70%					2.50%
Mortgages [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 2,609,829,000
Office Building [Member]
Debt Instrument [Line Items]
Interest Expense	22,869,000	$ 12,789,000	$ 7,006,000
total current additions [Member]
Debt Instrument [Line Items]
Long-term Debt	355,638,000
February 2017 facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 200,000,000				$ 90,000,000