Unsecured Revolving Line of Credit (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) numberofbanks	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 425,000,000	$ 400,000,000
Letters of credit outstanding, amount	200,000	0
Commercial paper issuances	0.0	319,556,000
LIBOR borrowings and letter of credit, amount outstanding	308,200,000	319,600,000
Net availability as of December 31, 2018	116,800,000	80,400,000
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 400,000,000	400,000,000
Number of institutions participating in the credit facility | numberofbanks	8
Number of institutions participating in the credit faciltiy pertaining to maximum contributory percentage | numberofbanks	1
Line of credit facility, maximum percentage of total availability provided by a single lender	16.00%
Commitment fees	$ 400,000	500,000
Line of credit facility, expiration date	Dec. 12, 2021
Swingline Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 25,000,000	0
Line of credit facility, expiration date	Mar. 27, 2020
LIBOR Borrowings [Member]
Line of Credit Facility [Line Items]
LIBOR borrowings	$ 308,000,000	$ 0.0
Unsecured Revolving Line Of Credit, Maximun Outstanding Under Accordian Feature [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 450,000,000
Eurodollar [Member] | Swingline Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.65%
Eurodollar [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.88%
Eurodollar [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Measurement Input, Credit Spread [Member] | Swingline Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.13%
Measurement Input, Credit Spread [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Measurement Input, Credit Spread [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%