Fair Value Measurements Narrative (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities:	$ 993,166,000	$ 978,284,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities:	993,166,000	978,284,000
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