Indebtedness (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsidiary Debt
Debt Instrument [Line Items]
Face Value				$ 5,554
Carrying Value	[1]			5,554	$ 81,752
Repayments in 2015		$ 2,000	$ 4,000
Senior Notes and Other Debt
Debt Instrument [Line Items]
Face Value				1,772,708
Carrying Value				$ 1,760,595	1,844,621
Senior Notes | Senior Notes due August 15, 2019
Debt Instrument [Line Items]
Rate				6.15%
Face Value				$ 140,651
Carrying Value				$ 140,301	149,484
Senior Notes | Senior Notes due September 15, 2019
Debt Instrument [Line Items]
Rate				7.375%
Face Value				$ 300,000
Carrying Value				$ 299,308	299,054
Senior Notes | Senior Notes due September 15, 2020
Debt Instrument [Line Items]
Rate				5.375%
Face Value				$ 300,000
Carrying Value				$ 298,747	298,411
Senior Notes | Senior Notes due January 1, 2022
Debt Instrument [Line Items]
Rate				8.70%
Face Value				$ 76,503
Carrying Value				$ 76,151	76,097
Senior Notes | Senior Notes due March 15, 2022
Debt Instrument [Line Items]
Rate				4.625%
Face Value				$ 350,000
Carrying Value				$ 347,834	347,417
Senior Notes | Senior Notes due February 15, 2037
Debt Instrument [Line Items]
Rate				6.25%
Face Value				$ 250,000
Carrying Value				$ 247,786	247,676
Senior Notes | Senior Notes due August 1, 2044 [Member]
Debt Instrument [Line Items]
Rate				4.75%
Face Value				$ 350,000
Carrying Value				344,914	344,730
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Aggregate Indebtedness				290,000	0
Face Value				110,000	0
Carrying Value				$ 727,630	340,320
Junior Subordinated Debt [Member] | Five Point Six Two Five Percent Subordinated Debentures [Member]
Debt Instrument [Line Items]
Rate				5.625%
Junior Subordinated Debt [Member] | Five Point Nine Percent Subordinated Debentures [Domain]
Debt Instrument [Line Items]
Rate				5.90%
Junior Subordinated Debt [Member] | Five Point Eight Percent Subordinated Debentures [Member] [Domain]
Debt Instrument [Line Items]
Rate				5.75%
Subordinated Debt [Member]
Debt Instrument [Line Items]
Face Value				$ 350,000
Subordinated Debt [Member] | SubordinatedDebenturedueApril30,2053 [Member]
Debt Instrument [Line Items]
Carrying Value				340,579
Subordinated Debt [Member] | SubordinatedDebenturedueMarch1,2056 [Member] [Member]
Debt Instrument [Line Items]
Carrying Value				105,952
Subordinated Debt [Member] | Subordinated Debenture due June1, 2056 [Member]
Debt Instrument [Line Items]
Carrying Value				281,099
Subordinated Debentures Subject to Mandatory Redemption [Member]
Debt Instrument [Line Items]
Face Value				$ 750,000	$ 340,320

[1] Subsidiary debt is due as follows: $4 million in 2017