Unsecured Revolving Line of Credit (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) numberofbanks	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, expiration date	Dec. 12, 2021
Unsecured Revolving Line Of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 400.0
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	$ 0.4	$ 0.4
Letters of credit outstanding, amount	$ 0.0
Maximum ratio of indebtedness to net capital threshold percentage	65.00%
Unsecured Revolving Line Of Credit, Maximun Outstanding Under Accordian Feature [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 450.0
Eurodollar [Member] | Minimum [Member] | Unsecured Revolving Line Of Credit [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.875%
Eurodollar [Member] | Maximum [Member] | Unsecured Revolving Line Of Credit [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%