Investments Allowance for Credit Losses - Aging of Past-Due Receivables (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Recorded investment in past due finance receivables on nonaccrual status	$ 0.5		$ 0.6
Recorded investment in impaired notes receivable, past due	$ 4.1		$ 4.1
Minimum [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Period Required For Considering Financing Receivable As Non Accrual Status	60 days
Past Due Period Of Financing Receivable Accruing Interest			90 days
Period for placing financing receivables on non-accrual status			89 days
Maximum [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Period Required For Considering Financing Receivable As Non Accrual Status		89 day