Reinsurance - Coverage and retention (Details) - Variable Interest Entity, Not Primary Beneficiary $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($)
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	$ 6,626,664
Current Coverage	4,649,174
Remaining Retention, Net	$ 1,864,823
Amortization period	10 years
Bellemeade 2017-1 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	$ 368,114	[1]
Current Coverage	108,368	[1]
Remaining Retention, Net	125,926	[1]
Bellemeade 2018-1 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	374,460	[2]
Current Coverage	181,136	[2]
Remaining Retention, Net	122,118	[2]
Bellemeade 2018-3 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	506,110	[3]
Current Coverage	302,563	[3]
Remaining Retention, Net	128,020	[3]
Bellemeade 2019-1 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	341,790	[4]
Current Coverage	181,324	[4]
Remaining Retention, Net	97,845	[4]
Bellemeade 2019-2 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	621,022	[5]
Current Coverage	398,316	[5]
Remaining Retention, Net	156,419	[5]
Bellemeade 2019-3 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	700,920	[6]
Current Coverage	409,859	[6]
Remaining Retention, Net	181,583	[6]
Bellemeade 2019-4 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	577,267	[7]
Current Coverage	411,954	[7]
Remaining Retention, Net	116,292	[7]
Bellemeade 2020-2 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	449,167	[8]
Current Coverage	218,877	[8]
Remaining Retention, Net	222,567	[8]
Bellemeade 2020-2 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	423,000
Bellemeade 2020-2 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	26,000
Bellemeade 2020-3 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	451,816	[9]
Current Coverage	365,141	[9]
Remaining Retention, Net	154,732	[9]
Bellemeade 2020-3 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	418,000
Bellemeade 2020-3 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	34,000
Bellemeade 2020-4 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	337,013	[10]
Current Coverage	186,065	[10]
Remaining Retention, Net	129,824	[10]
Bellemeade 2020-4 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	321,000
Bellemeade 2020-4 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	16,000
Bellemeade 2021-1 Ltd Member
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	643,577	[11]
Current Coverage	630,163	[11]
Remaining Retention, Net	149,189	[11]
Bellemeade 2021-1 Ltd Member | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	580,000
Bellemeade 2021-1 Ltd Member | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	64,000
Bellemeade 2021-2 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	616,017	[12]
Current Coverage	616,017	[12]
Remaining Retention, Net	139,763	[12]
Bellemeade 2021-2 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	523,000
Bellemeade 2021-2 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	93,000
Bellemeade 2021-3 Ltd
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	639,391	[13]
Current Coverage	639,391	[13]
Remaining Retention, Net	140,545	[13]
Bellemeade 2021-3 Ltd | Funded By VIE
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	508,000
Bellemeade 2021-3 Ltd | Directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage at Issuance	$ 131,000

[1] Issued in October 2017, covering in-force policies issued between January 1, 2017 and June 30, 2017.

[2] Issued in April 2018, covering in-force policies issued between July 1, 2017 and December 31, 2017.

[3] Issued in October 2018, covering in-force policies issued between January 1, 2018 and June 30, 2018.

[4] Issued in March 2019, covering in-force policies primarily issued between 2005 to 2008 under United Guaranty Residential Insurance Company (“UGRIC”); as well as policies issued through 2015 under both UGRIC and Arch Mortgage Insurance Company.

[5] Issued in April 2019, covering in-force policies issued between July 1, 2018 and December 31, 2018.

[6] Issued in July 2019, covering in-force policies issued in 2016.

[7] Issued in October 2019, covering in-force policies issued between January 1, 2019 and June 30, 2019.

[8] Issued in September 2020, covering in-force policies issued between January 1, 2020 and May 31, 2020. $423 million was directly funded by Bellemeade 2020-2 Ltd. with an additional $26 million of capacity provided directly to Arch MI U.S. by a separate panel of reinsurers

[9] Issued in November 2020, covering in-force policies issued between June 1, 2020 and August 31, 2020. $418 million was directly funded by Bellemeade 2020-3 Ltd. with an additional $34 million of capacity provided directly to Arch MI U.S. by a separate panel of reinsurers.

[10] Issued in December 2020, covering in-force policies issued between July 1, 2019 and December 31, 2019. $321 million was directly funded by Bellemeade 2020-4 Ltd. with an additional $16 million of capacity provided directly to Arch MI U.S. by a separate panel of reinsurers

[11] Issued in March 2021, covering in-force policies issued between September 1, 2020 and November 30, 2020. $580 million was directly funded by Bellemeade Re 2021-1 Ltd. with an additional $64 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers.

[12] Issued in June 2021, covering in-force policies issued between December 1, 2020 and March 31, 2021. $523 million was directly funded by Bellemeade Re 2021-2 Ltd. via insurance-linked notes, with an additional $93 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers

[13] Issued in September 2021, covering in-force policies issued between April 1, 2021 and June 30, 2021. $508 million was directly funded by Bellemeade Re 2021-3 Ltd. via insurance-linked notes, with an additional $131 million capacity provided directly to Arch MI U.S. by a separate panel of reinsurers