Composition of Certain Financial Statement Captions (Deferred Contract Costs) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Items Included in Consolidated Statement of Financial Condition [Abstract]
Deferred contract costs	$ 33,833,000		$ 33,833,000		$ 30,991,000
Capitalized contract cost, amount capitalized during period	$ 2,521,000	$ 4,575,000	7,270,000	$ 7,673,000
Amortization of deferred contract costs			4,428,000	$ 3,174,000
Capitalized contract cost impairment			$ 0