Fair Value - Financial Statement Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Short-term investments at fair value	$ 90,663	$ 117,824
Loans held for sale	4,522,971	2,521,647
Derivative assets:
Derivative asset, before netting	198,391	123,316
Netting	34,557	(26,969)
Total derivative assets	232,948	96,347
Mortgage servicing rights at fair value	2,550,602	2,807,333
Derivative liabilities:
Derivative liability, before netting	88,359	28,146
Netting	(74,324)	(25,082)
Net amounts of liabilities presented in the consolidated balance sheet	14,035	3,064
Mortgage servicing liabilities	34,294	8,681
PMT
Derivative assets:
Investment in PennyMac Mortgage Investment Trust	1,667	1,397
Interest rate lock commitments
Derivative assets:
Total derivative assets	147,400	50,507
Forward contracts | Purchases
Derivative assets:
Derivative asset, before netting	17,943	35,916
Derivative liabilities:
Derivative liability, before netting	51,585	215
Forward contracts | Sales
Derivative assets:
Derivative asset, before netting	6,141	437
Derivative liabilities:
Derivative liability, before netting	34,498	26,762
MBS put options
Derivative assets:
Derivative asset, before netting	10,040	720
MBS call options
Derivative assets:
Derivative asset, before netting		2,135
Call options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	2,414	5,965
Put options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	6,266	866
Level 3 | Interest rate lock commitments
Assets:
Loans held for sale	145,124	49,338
Recurring basis
Assets:
Short-term investments at fair value	90,663	117,824
Loans held for sale	4,522,971	2,521,647
Derivative assets:
Derivative asset, before netting	198,391	123,316
Netting	34,557	(26,969)
Total derivative assets	232,948	96,347
Mortgage servicing rights at fair value	2,556,253	2,820,612
Total assets	7,404,502	5,557,827
Derivative liabilities:
Derivative liability, before netting	88,359	28,146
Netting	(74,324)	(25,082)
Net amounts of liabilities presented in the consolidated balance sheet	14,035	3,064
Mortgage servicing liabilities	34,294	8,681
Total liabilities	231,470	227,855
Recurring basis | PMT
Derivative assets:
Investment in PennyMac Mortgage Investment Trust	1,667	1,397
Derivative liabilities:
Excess servicing spread financing at fair value to affiliate	183,141	216,110
Recurring basis | Interest rate lock commitments
Derivative assets:
Derivative asset, before netting	147,400	50,507
Derivative liabilities:
Derivative liability, before netting	2,276	1,169
Recurring basis | Repurchase agreement derivatives
Derivative assets:
Derivative asset, before netting	8,187	26,770
Recurring basis | Forward contracts | Purchases
Derivative assets:
Derivative asset, before netting	17,943	35,916
Derivative liabilities:
Derivative liability, before netting	51,585	215
Recurring basis | Forward contracts | Sales
Derivative assets:
Derivative asset, before netting	6,141	437
Derivative liabilities:
Derivative liability, before netting	34,498	26,762
Recurring basis | MBS put options
Derivative assets:
Derivative asset, before netting	10,040	720
Recurring basis | MBS call options
Derivative assets:
Derivative asset, before netting		2,135
Recurring basis | Call options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	2,414	5,965
Recurring basis | Put options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	6,266	866
Recurring basis | Level 1
Assets:
Short-term investments at fair value	90,663	117,824
Derivative assets:
Derivative asset, before netting	8,680	6,831
Total derivative assets	8,680	6,831
Total assets	101,010	126,052
Recurring basis | Level 1 | PMT
Derivative assets:
Investment in PennyMac Mortgage Investment Trust	1,667	1,397
Recurring basis | Level 1 | Call options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	2,414	5,965
Recurring basis | Level 1 | Put options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	6,266	866
Recurring basis | Level 2
Assets:
Loans held for sale	4,437,954	2,261,639
Derivative assets:
Derivative asset, before netting	34,124	39,208
Total derivative assets	34,124	39,208
Total assets	4,472,078	2,300,847
Derivative liabilities:
Derivative liability, before netting	86,083	26,977
Net amounts of liabilities presented in the consolidated balance sheet	86,083	26,977
Total liabilities	86,083	26,977
Recurring basis | Level 2 | Forward contracts | Purchases
Derivative assets:
Derivative asset, before netting	17,943	35,916
Derivative liabilities:
Derivative liability, before netting	51,585	215
Recurring basis | Level 2 | Forward contracts | Sales
Derivative assets:
Derivative asset, before netting	6,141	437
Derivative liabilities:
Derivative liability, before netting	34,498	26,762
Recurring basis | Level 2 | MBS put options
Derivative assets:
Derivative asset, before netting	10,040	720
Recurring basis | Level 2 | MBS call options
Derivative assets:
Derivative asset, before netting		2,135
Recurring basis | Level 3
Assets:
Loans held for sale	85,017	260,008
Derivative assets:
Derivative asset, before netting	155,587	77,277
Total derivative assets	155,587	77,277
Mortgage servicing rights at fair value	2,556,253	2,820,612
Total assets	2,796,857	3,157,897
Derivative liabilities:
Derivative liability, before netting	2,276	1,169
Net amounts of liabilities presented in the consolidated balance sheet	2,276	1,169
Mortgage servicing liabilities	34,294	8,681
Total liabilities	219,711	225,960
Recurring basis | Level 3 | PMT
Derivative liabilities:
Excess servicing spread financing at fair value to affiliate	183,141	216,110
Recurring basis | Level 3 | Interest rate lock commitments
Derivative assets:
Derivative asset, before netting	147,400	50,507
Derivative liabilities:
Derivative liability, before netting	2,276	1,169
Recurring basis | Level 3 | Repurchase agreement derivatives
Derivative assets:
Derivative asset, before netting	$ 8,187	$ 26,770