Senior Unsecured Notes - Reconciliation of Senior Unsecured Notes to Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Senior unsecured notes, beginning balance	$ 944,780
Amortization of deferred financing costs	504	$ 557
Senior unsecured notes, ending balance	945,173		$ 944,780
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Senior unsecured notes, beginning balance	944,780	$ 944,088	944,088
Accretion of discount	770		1,539
Amortization of note premium	(531)		(1,062)
Amortization of deferred financing costs	154		312
Senior unsecured notes, ending balance	$ 945,173		944,780
Senior Unsecured Notes [Member] \| 5.6250% Senior Notes [Member]
Debt Instrument [Line Items]
Additional deferred financing costs			$ (97)

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration