Piper Jaffray Companies (Parent Company only) - Condensed Statements of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Interest									$ 31,954	$ 33,074	$ 41,557
Investment income/(loss)									18,002	24,602	10,736
Total revenues	$ 240,782	$ 244,915	$ 204,007	$ 205,487	$ 227,605	$ 206,276	$ 176,392	$ 159,601	895,191	769,874	696,317
Net revenues	236,082	240,567	197,745	200,529	222,463	200,847	170,483	153,556	874,923	747,349	672,918
Non-interest expenses:
Total non-interest expenses	225,845	322,803	177,878	177,720	281,739	182,396	163,974	150,114	904,246	778,223	586,495
Income tax expense	57,141	(31,423)	4,906	(395)	(25,895)	6,515	1,996	256	30,229	(17,128)	27,941
Income of parent company	46,904	50,813	(14,961)	(23,204)	33,381	(11,936)	(4,513)	(3,186)	59,552	13,746	(58,482)
Net income/(loss) applicable to Piper Jaffray Companies	$ (46,074)	$ (49,713)	$ 13,573	$ 20,275	$ (36,985)	$ 10,658	$ 1,938	$ 2,437	(61,939)	(21,952)	52,075
Parent Company
Revenues:
Dividends from subsidiaries									120,102	104,016	37,649
Interest									1,125	994	650
Investment income/(loss)									4,060	1,835	(2,033)
Total revenues									125,287	106,845	36,266
Interest expense									7,170	8,195	6,406
Net revenues									118,117	98,650	29,860
Non-interest expenses:
Total non-interest expenses									4,936	4,505	3,487
Income before income tax expense and equity in income of subsidiaries									113,181	94,145	26,373
Income tax expense									35,589	27,952	9,191
Income of parent company									(77,592)	(66,193)	(17,182)
Equity in undistributed/(distributed in excess of) income of subsidiaries									(139,531)	(88,145)	34,893
Net income/(loss) applicable to Piper Jaffray Companies									$ (61,939)	$ (21,952)	$ 52,075