Investments (Roll Forward of Cumulative Credit Losses Recorded in Earnings) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance beginning of period	$ 470	$ 93
Additional credit losses recognized during the period, related to securities for which, No impairment has been previously recognized	1,064	49
Balance March 31	$ 1,534	$ 142

Credit Losses Recognized In Earnings In Which A Portion Of Losses Are Recognized In Comprehensive Income (Loss), Credit Losses On Debt Securities Held

No definition available.

Name:	pra_CreditLossesRecognizedInEarningsInWhichAPortionOfLossesAreRecognizedInComprehensiveIncomeLossCreditLossesOnDebtSecuritiesHeld
Namespace Prefix:	pra_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant