Debt (Details) - USD ($)			3 Months Ended		12 Months Ended
	Jul. 02, 2018	Jun. 29, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Long-term Debt, Gross			$ 908,500,000		$ 909,000,000.0
Repayments of Debt					215,000,000.0
Debt Instrument, Collateral Amount			198,000,000.0		15,700,000
Interest Expense			11,500,000	$ 8,000,000.0
Interest Paid, Including Capitalized Interest, Operating and Investing Activities			19,800,000	12,500,000
Secured Debt [Member]
Debt Instrument [Line Items]
Proceeds from Debt, Net of Issuance Costs		$ 249,400,000
Senior Notes [Member] | Term Loan Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			34,900,000		34,900,000
Senior Notes [Member] | Senior Note Due on September Nineteen,Twenty Twenty Two [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			281,000,000.0		281,500,000
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			448,400,000		448,400,000
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross			144,200,000		144,200,000
Notes Payable under Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding						$ 0
Federal Home Loan Bank of Dallas [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank Stock			3,300,000
Federal Home Loan Bank of Chicago [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances					10,000,000.0
Federal Home Loan Bank Stock			3,800,000		800,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances			$ 187,700,000	$ 10,000,000.0	$ 10,000,000.0
Infinity [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Business Combination, Consideration Transferred, Liabilities Incurred	$ 250,000,000.0
Infinity [Member] | Senior Notes [Member] | Senior Note Due on September Nineteen,Twenty Twenty Two [Member]
Debt Instrument [Line Items]
Debt Instrument, Unamortized Premium	(7,100,000)
Debt Instrument, Face Amount	$ 275,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	5.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	$ 282,100,000
Debt Instrument, Interest Rate, Effective Percentage	4.36%