Net Interest Income (Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Interest income:
Short-term investments	$ 814	$ 733	$ 2,001	$ 1,727
Mortgage loans held for sale at fair value	34,941	28,199	95,982	68,528
Placement fees relating to custodial funds	23,397	13,394	55,014	27,073
Interest income, excluding related parties	59,152	42,326	152,997	97,328
Interest income	60,964	44,442	158,683	103,274
Interest expense:
Assets sold under agreements to repurchase	4,676	19,203	15,943	52,249
Mortgage loan participation purchase and sale agreements	2,533	1,484	6,450	3,780
Notes payable	21,369	11,747	55,939	24,746
Obligations under capital lease	122	205	444	585
Interest shortfall on repayments of mortgage loans serviced for Agency securitizations	4,883	4,602	14,259	11,529
Interest on mortgage loan impound deposits	1,452	1,253	3,517	2,943
Interest expense, non-affiliates	35,035	38,494	96,552	95,832
Interest expense	38,775	42,492	108,136	108,843
Net interest income (expense):	22,189	1,950	50,547	(5,569)
PMT
Interest income:
From PennyMac Mortgage Investment Trust	1,812	2,116	5,686	5,946
Interest expense:
To PennyMac Mortgage Investment Trust Excess servicing spread financing at fair value	3,740	3,998	11,584	13,011
Assets sold under agreements to repurchase
Interest expense:
Assets sold under agreements to repurchase	4,676	$ 19,203	15,943	$ 52,249
Incentives recorded	$ 12,800		$ 35,500
Repurchase agreement initial term					6 months