Fair Value Measurements Narrative (Details)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) security
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities:	$ 966,731,000	$ 994,065,000
Transfers of assets from Level 1 to Level 2, numbers of assets | security		5
Transfers of assets from Level 1 to Level 2, amount		$ 33,400,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities:	966,731,000	994,065,000
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities:	0	0
Assets value	$ 0	$ 0