Composition of Certain Financial Statement Captions (Deferred Contract Costs) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Items Included in Consolidated Statement of Financial Condition [Abstract]
Deferred contract costs	$ 34,263,000		$ 34,263,000		$ 33,781,000
Capitalized contract cost, amount capitalized during period	$ 2,932,000	$ 2,521,000	6,978,000	$ 7,270,000
Amortization of deferred contract costs			6,496,000	$ 4,428,000
Capitalized contract cost impairment			$ 0