Fair Value - Financial Statement Items Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Fair value
Interest rate threshold used in determination of accounting for loans underlying mortgage servicing rights (as a percent)	4.50%
Assets:
Short-term investments at fair value	$ 170,080	$ 117,824
Mortgage loans held for sale	3,099,103	2,521,647	$ 2,416,955	$ 2,527,231	$ 2,584,236	$ 2,935,593	$ 3,037,602	$ 2,277,751
Derivative assets:
Derivative asset, before netting	85,046	123,316
Netting	(6,867)	(26,969)
Total derivative assets	78,179	96,347
Mortgage servicing rights at fair value	638,010	2,820,612
Derivative liabilities:
Derivative liability, before netting	10,043	28,146
Netting	(4,247)	(25,082)
Net amounts of liabilities presented in the consolidated balance sheet	5,796	3,064
Mortgage servicing liabilities	14,120	8,681
PMT
Derivative assets:
Investment in PennyMac Mortgage Investment Trust	1,205	1,397
Interest rate lock commitments
Derivative assets:
Total derivative assets	60,012	50,507
Recurring basis
Assets:
Short-term investments at fair value	170,080	117,824
Mortgage loans held for sale	3,099,103	2,521,647
Derivative assets:
Derivative asset, before netting	85,046	123,316
Netting	(6,867)	(26,969)
Total derivative assets	78,179	96,347
Mortgage servicing rights at fair value	638,010	2,820,612
Total assets	3,986,577	5,557,827
Derivative liabilities:
Derivative liability, before netting	10,043	28,146
Netting	(4,247)	(25,082)
Net amounts of liabilities presented in the consolidated balance sheet	5,796	3,064
Mortgage servicing liabilities	14,120	8,681
Total liabilities	256,450	227,855
Recurring basis | PMT
Derivative assets:
Investment in PennyMac Mortgage Investment Trust	1,205	1,397
Derivative liabilities:
Excess servicing spread financing at fair value to affiliate	236,534	216,110
Recurring basis | Repurchase agreement derivatives
Derivative assets:
Derivative asset, before netting	10,656
Recurring basis | MBS call options
Derivative assets:
Derivative asset, before netting		2,135
Recurring basis | Interest rate lock commitments
Derivative assets:
Derivative asset, before netting	60,012	50,507
Derivative liabilities:
Derivative liability, before netting	1,740	1,169
Recurring basis | Repurchase agreement derivatives
Derivative assets:
Derivative asset, before netting		26,770
Recurring basis | Forward contracts | Purchases
Derivative assets:
Derivative asset, before netting	4,288	35,916
Derivative liabilities:
Derivative liability, before netting	1,272	215
Recurring basis | Forward contracts | Sales
Derivative assets:
Derivative asset, before netting	2,101	437
Derivative liabilities:
Derivative liability, before netting	7,031	26,762
Recurring basis | MBS put options
Derivative assets:
Derivative asset, before netting	3,481	720
Recurring basis | Call options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	938	5,965
Recurring basis | Put options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	3,570	866
Recurring basis | Level 1
Assets:
Short-term investments at fair value	170,080	117,824
Derivative assets:
Derivative asset, before netting	4,508	6,831
Total derivative assets	4,508	6,831
Total assets	175,793	126,052
Recurring basis | Level 1 | PMT
Derivative assets:
Investment in PennyMac Mortgage Investment Trust	1,205	1,397
Recurring basis | Level 1 | Call options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	938	5,965
Recurring basis | Level 1 | Put options on interest rate futures | Purchases
Derivative assets:
Derivative asset, before netting	3,570	866
Recurring basis | Level 2
Assets:
Mortgage loans held for sale	2,316,892	2,261,639
Derivative assets:
Derivative asset, before netting	9,870	39,208
Total derivative assets	9,870	39,208
Total assets	2,326,762	2,300,847
Derivative liabilities:
Derivative liability, before netting	8,303	26,977
Net amounts of liabilities presented in the consolidated balance sheet	8,303	26,977
Total liabilities	8,303	26,977
Recurring basis | Level 2 | MBS call options
Derivative assets:
Derivative asset, before netting		2,135
Recurring basis | Level 2 | Forward contracts | Purchases
Derivative assets:
Derivative asset, before netting	4,288	35,916
Derivative liabilities:
Derivative liability, before netting	1,272	215
Recurring basis | Level 2 | Forward contracts | Sales
Derivative assets:
Derivative asset, before netting	2,101	437
Derivative liabilities:
Derivative liability, before netting	7,031	26,762
Recurring basis | Level 2 | MBS put options
Derivative assets:
Derivative asset, before netting	3,481	720
Recurring basis | Level 3
Assets:
Mortgage loans held for sale	782,211	260,008
Derivative assets:
Derivative asset, before netting	70,668	77,277
Total derivative assets	70,668	77,277
Mortgage servicing rights at fair value	638,010	2,820,612
Total assets	1,490,889	3,157,897
Derivative liabilities:
Derivative liability, before netting	1,740	1,169
Net amounts of liabilities presented in the consolidated balance sheet	1,740	1,169
Mortgage servicing liabilities	14,120	8,681
Total liabilities	252,394	225,960
Recurring basis | Level 3 | PMT
Derivative liabilities:
Excess servicing spread financing at fair value to affiliate	236,534	216,110
Recurring basis | Level 3 | Repurchase agreement derivatives
Derivative assets:
Derivative asset, before netting	10,656
Recurring basis | Level 3 | Interest rate lock commitments
Derivative assets:
Derivative asset, before netting	60,012	50,507
Derivative liabilities:
Derivative liability, before netting	$ 1,740	1,169
Recurring basis | Level 3 | Repurchase agreement derivatives
Derivative assets:
Derivative asset, before netting		$ 26,770