Revenues from Contracts with Customers (Schedule of Total Revenues Separated between Revenues from Contracts with Customers and Other Sources of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Revenue from contracts with customers									$ 2,187,145
Other									15,568
Total revenue from contracts with customers									2,187,145
Other sources of revenue:
Interest	$ 184,534	$ 169,760	$ 154,421	$ 137,734	$ 126,615	$ 117,862	$ 108,951	$ 100,953	646,449	$ 454,381	$ 294,332
Principal transactions									351,378	396,826	475,428
Other									9,985
Total revenues	$ 851,325	$ 786,810	$ 780,011	$ 776,811	$ 823,950	$ 738,794	$ 742,291	$ 691,427	3,194,957	2,996,462	2,642,370
Commissions [Member]
Revenues:
Revenue from contracts with customers									657,732	678,904	729,989
Investment Banking [Member]
Revenues:
Revenue from contracts with customers									707,670	726,763	513,034
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers									$ 806,175	$ 702,064	$ 582,789