Investments (Narrative) (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) security	Mar. 31, 2019 USD ($) security	Dec. 31, 2019 security
Schedule of Available-for-sale Securities [Line Items]
Gross gains on investments	$ 20,400,000	$ 4,200,000
Gross losses on investments	14,800,000	100,000
Proceeds from sale of investment securities	100,800,000	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Number of Positions [Abstract]
Other-than-temporary impairment	$ 0	0
Realized gains on sale of securities		$ 4,100,000
Number of securities sold | security		6
Debt Securities [Member]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Number of Positions [Abstract]
Number of debt securities with unrealized loss | security	167		61
Number of debt securities with unrealized loss, less than 12 months | security	150		41
Number of debt securities with unrealized loss, greater than 12 months | security	17		20